Showing papers in "Surface Engineering in 1996"

Chemical potentials and activities of nitrogen and carbon imposed by gaseous nitriding and carburising atmospheres

Abstract: The nitrogen and carbon activities are the cardinal parameters for process control of nitriding, nitrocarburising, carburising, and carbonitriding. The essential elements of the thermodynamic background for the definition of the so called nitrogen and carbon activities in nitrided and carburised surface layers of iron based substrates are presented in a comparative manner. Choice of the distinct reference states is discussed in relation to the consequences for the interpretation of activities and equilibrium constants of the reactions. Practical examples are used to show how the nitrogen and carbon activities can be established; the activities can, for example, be used to determine the amounts of dissolved nitrogen and carbon. Also, knowledge of the nitrogen and carbon activities makes it possible to minimise and to control nitrogen gas pore formation and/or to avoid cementite formation in engineered surface layers.

Laser metal forming: process fundamentals

Abstract: Laser metal fornling (LMF) is an innovative application for lasers with industrial potential for small series production, rapid prototyping, or repair engineering. It consists in the production of full strength metallic engineering components using a three-dimensional laser cladding process. Prototypes produced by LMF can be used to test the performance and reliability aspects of a particular design and must therefore have physical and mechanical properties similar to those of the mass produced part. The LMF technique can also be used to repair metallic parts and improve their life cycle behaviour. The fundamental aspects of this free forming technique were investigated to contribute to the physical understanding and to better control of the process. To this end, only simple geometrical parts have been produced. It was found that the dimensions, the shape, the shape instabilities, and the surface quality of the final components depend on the melt dynamics and on the kinetics of oxide formation at ...

S phase coatings produced by unbalanced magnetron sputtering

Abstract: The wear resistance of austenitic stainless steels can not be adequately improved by conventional plasma nitriding without compromising the corrosion resistance. However, recently a phase, known by some as‘S phase’, based on an alloy of austenitic stainless steel and nitrogen, has shown potential for simultaneously improving wear and corrosion resistance. Coatings of this material were deposited on copper and glass substrates by the reactive magnetron sputtering of AISI 316 targets under various nitrogen partial pressures. The coatings deposited using a single unbalanced magnetron all consisted of fine, spherically tipped columnar grains and showed a characteristic broad X-ray diffraction peak between 45 and 55° 2θ. The hardness of these coatings on copper substrates was found to have a maximum value of ∼700 HV at ∼ O.5 mtorr (∼ O.7 ¨bar) nitrogen partial pressure. One coating deposited on glass using two unbalanced magnetrons operating in closed field mode differed from the other coatings, consis...

Deflection measurements as method to determine residual stress in thin hard coatings on tool materials

Abstract: Current techniques for residual stress measurement can be divided into two general classes: those based on direct measurelnent of the elastic strains in the film using X -ray diffraction; and those based on the associated curvature or deflection of a thin substrate. The most widely used deflection technique is to coat a relatively thin silicon substrate and then measure the curvature of radius resulting from the coating residual stress. However, since this residual stress strongly depends on the substrate material, it is desirable to develop a coating stress measurement technique applicable to any coated material. Thus, the purpose of the present work was to study the feasibility of the deflection technique for residual stress measurements of thin hard coatings on tool materials. This means, in practice, obtaining a suitable technique for mechanical thinning of coated tool materials. Residual stress values obtained using X-ray diffraction data (sin2 ψ method) and literature values of the coating e...

Corrosion behaviour and microstructure of two thermal spray coatings

Abstract: Many of the state of the art thermal spray coatings (e.g. plasma, HVOF) have been developed with wear resistance as a primary aim. However, these coatings are increasingly being required to functio...

EFFECT OF RARE EARTH ELEMENTS ON PLASMA NITRIDING OF 38CrMoAI STEEL

Abstract: Plaslna nitriding of steels has been intensively investigated and has found many applications in several industrial sectors owing to the excellent combination of properties obtained, i.e. good tribological performance and high fatigue strength. Although some preliminary research work has reported that rare earth elements can speed up most thermochemical treatments, little attention has been paid to the effect of these elements on the plasma assisted thermochemical processes. In the present investigation, plasma nitriding of 38CrMoAl steel has been carried out in order to investigate the effect of introducing rare earth elements on the process and on the structures and abrasive wear resistance of the resultant sUljace layers. Systematic characterisation of the surface layers has been perfonned elnploying optical and electron microscopy (morphology of worn surface), X-ray diffraction (phase identification), energy dispersive X-ray analysis (composition), and microhardness testing. Abrasive wear test...

STRUCTURAL CHARACTERISATION OF NOVEL Ti-Cu INTERMETALLIC COATINGS

Abstract: The structure and composition of an innovative Ti-Cu intermetallic coating has been investigated by means of metallographic examination, X-ray diffraction, glow discharge optical spectroscopy, and microhardness measurements. For this purpose, Ti-6Al-4V substrates were coated with a uniform layer of copper by an electroplating technique, then a diffusion anneal was conducted in an argon controlled atmosphere. Microstructural examination revealed that the compound coating microstructure consisted of four layers: the outer two layers are composed of various intermetallic compounds, including TiCu3 + Ti3 Al and TiCu; the third layer is the transformation product of the β phase electroplatformed at the diffusion temperature, which contains α- Ti and Ti2Cu; and the fourth layer was identified as the product of this transformation precipitated mainly at proeutectoid (α- Ti boundaries. These microstructural features reveal the two essential characteristics of the coatings, namely the continuous layers of ...

Interfacial fatigue limit as measure of cyclic bonding strength of hard coatings

Abstract: An interfacial fatigue test technique has been developed to evaluate the contact fatigue behaviour of various coating-substrate systems. It is shown that failure of the coated systems is initiated at the interface and that the intelfacial fatigue limit is a sensitive measure of the bonding condition between the coating and substrate which remains insensitive to the hardness of both materials and the testing conditions. This characteristic of the test is significant in that it allows the separation of bonding strength from other properties of the coating-substrate system. The nature of the test is investigated theoretically in terms of contact mechanics of both homogeneous and layered materials. It is revealed that the shear stress range at the interface is an appropriate parameter for comparing experimental results. Using the critical shear stress range at the interface for coating debonding, the cyclic bonding strengths of various coating-substrate systems can be quantitatively evaluated and comp...

Modelling of austenite formation during rapid heating

Abstract: Numerical models of austenite formation during rapid heating of steels are presented. Shifted transformation temperatures, diffusion controlled growth, impingement effects, and the possibility of diffusion less growth are considered. Local equilibrium at the phase interfaces is assumed. Further, the effect of alloying elements under paraequilibrium conditions is taken into account. The model is applied to electron beam hardening of hypoeutectoid steels in order to predict the properties of the hardened zone.

Electron beam surface engineering of aluminium bronze

Abstract: Electron beam surface melting has been carried out on a complex aluminium bronze, CA104. The structural changes have been studied using optical microscopy, scanning and transmission electron microscopy, and X -ray diffraction analysis. The effect of surface melting on hardness, rolling–sliding wear resistance and friction coefficient, and electrochemical corrosion properties has been investigated. It was found that the surface melting transformed the original α + K structure into a martensitic one. Consequently, the hardness was almost doubled in the melted region. Subsequent tempering further increased the hardness. When running against a hardened steel in dry rolling–sliding wear both the weight loss and the coefficient of friction were reduced by surface melting. Electrochemical corrosion tests also showed an improvement in corrosion resistance in simulated cooling water after surface treatment.

Microstructure and properties of laser remelted cobalt based and nickel based plasma sprayed coatings

Abstract: The effect of laser remelting on the microstructure, porosity, and chemical homogeneity of cobalt based and nickel based alloy coatings plasma sprayed on stainless steel substrates has been investigated. Coating, nicrostructure and density were markedly improved by laser melting and much better chemical homogeneity was obtained. The micro hardness and corrosion resistance of the coatings were also enhanced significantly following laser melting.

Phase selection during solidification of AISI 420 and AISI 440C tool steels

Abstract: In the present work the influence of solidification conditions on the primary solidification modes of two martensitic stainless tool steels (AISI 420 and AISI 440C) is analysed, with the aim of establishing a basis for the selection of the laser treatment parameters that lead to the most favourable microstructures. Kurz, Giovanola, and Trivedi s solidification model is used to analyse the competition between δferrite and austenite during solidification. The theoretical calculations are compared with experimental results obtained after laser surface melting of AISI420 and AISI 440C with scanning speeds from 1 × 10−3 to 2 × 10−1 m s−l. Calculated results are in good agreement with experimental observations and it is shown that, although the equilibrium solidification mode of both steels used is δferrite, the precipitation of primary austenite from the liquid is kinetically favoured.

New method for low temperature gaseous nitriding of iron

Abstract: A new method for obtaining nitride layers in iron at low temperatures is presented. In this method, bulk iron is coated with a thin layer of nickel and is then treated in an NH3/H2 atmosphere at a temperature of 325°C. Analysis by elastic recoil detection, X-ray diffraction, and Rutherford backscattering spectrometry shows that nitride layers are formed in Fe which is covered with a Ni layer, while uncovered Fe is oxidised by impurities in the gas. The influence of the Ni layer is discussed. It is shown that it is possible to obtain pore free iron nitride layers at low temperatures.

Factors determining growth of diffusion layers on low carbon steel and iron during vacuum chromising

Abstract: Layers produced on low carbon steel and Armco iron during vacuum chromising have been investigated. In particular, the influence of two fundamental factors, the surface area of the sublimating metallic chromium and the pressure within the furnace chamber, is discussed. Layer thickness and morphology, and the chromium and iron concentrations in the diffusion zone of chromised layers, have been studied. Layers consisting of solid solutions of chromium in iron resulted in all instances. Chromising conducted at low pressure, with a static rather than a dynamic vacuum, was found to be most effective for maximising the rate of growth of diffusion layers. Scanning electron micrographs of the surfaces of chromised low carbon iron alloys show the presence of microsteps characteristic of screw dislocations. Hence, it is concluded that the growth of diffusion layers during vacuum chromising depends on the generation and growth of dislocations according to Frank‘s spiral lnechanism model.

MICROSTRUCTURE AND SLIDING WEAR PROPERTIES OF LASER CLAD TiN REINFORCED COMPOSITE COATING

Abstract: The microstructure and sliding wear properties of a laser clad TiN composite coating on steel 1045 have been investigated using scanning electron microscopy, electron probe microanalysis, X -ray diffraction, and energy dispersive X-ray analysis and a pin-ring dry sliding wear test machine. Excellent fusion bonding with low dilution and absence of cracks has been obtained under laser cladding conditions in which specific energy was in the range 5-20 kJ cm-2 and power density was in the range 3·5-5 kW cm-2. The clad layers have been characterised by metallurgical examination, which showed that the bonding zone, which is about 6 ¨m thick, is a γ nickel solid solution growing from the bottom of the molten pool with planar crystal morphology. Particles of TiN, γ nickel, and a fine eutectic of γ nickel and (Fe, Cr)23 C6 in the interdendritic regions have been observed and a large number of TiN particles with irregular shapes remained after laser cladding. Partial dissolution occurred on the edges of the...

Surface engineering of steels by plasma immersion ion implantation and related innovative techniques

Abstract: Surface engineering of steels has been carried out using plasma inlmersion ion implantation (PI3) and related innovative techniques including plasma immersion ion beam enhanced deposition (PIIBED) of titanium and plasma immersion ion beam mixing (PIIBM) of nitrogen and titanium. Auger concentration profiles indicate that PI3 of nitrogen produces a smooth implantation profile in the surface layer of the steel. The formation of a thin layer of TiN on the surface of the specimens treated by PIIBED of titanium and by PIIBM of nitrogen and titanium was confirmed by X-ray photoelectron spectroscopy. It has been demonstrated that the surface hardness and wear resistance of all the treated specimens were greatly improved.

Effect of laser surface hardening on permeability of hydrogen through 1045 steel

Abstract: The effect of laser surface hardening on hydrogen permeability through AISI 1045 steel was measured and determination of the contribution of compressive stresses to this effect was attempted. Metallography and micro hardness measurements of the heat affected zone were performed to determine the depth of the most hardened zone. It was found that the steady state permeability of hydrogen through steel was reduced from 1·19 x 10-6 A cm-2in a normalised condition to 0·38 x 10-6 A cm-2 for a laser hardened specimen. But when compressive stresses in the hardened layer were relieved by cutting the layer off, the permeation rate of hydrogen increased to 1·74 x 10-6 A cm-2. After correctionfor the effect of membrane thickness on steady state permeability, and analysis of the effective diffusivity using Fick s first diffusion law, it was concluded that the effect can be attributed to compressive stresses in the hardened layer. It was also found that martensite reduces the effective diffusivity of hydrogen b...

Residual stress characterisation of grit blasted steel surfaces

Abstract: With most surface engineering procedures it is necessary to create good initial surface conditions. In the case of thermal spraying processes it is recommended that a rough surface is created to enhance adhesion of the coating, especially when the coating contains ceramics. For optimum mechanical properties, in particular fatigue behaviour, the surface must remain in a compressive stress state. When high velocity oxyfuel spraying is used a compressive residual stress condition is strongly advised in view of the pressure and velocity with which the powder particles reach the substrate. The method of surface preparation employed can strongly affect the resultant compressive stresses. The effects of grit blasting conditions on the resultant residual stress state have been studied. Compressive residual stress in the surface layer increases with grit blasting time. Time, gun-specimen distance, and impingement angle have also been evaluated and show complementary behaviour.

Production of duplex coatings by thermal spraying and nitriding

Abstract: Limitations of surface treatments can be apparent under the complex loading experienced by engineering parts and components. These limitations can be removed by using duplex techniques. The process combination of thermal spraying and nitriding makes it possible to produce coatings with increased resistance for applications involving complex stress and loading conditions. The interactions between thermal spraying and nitriding have been investigated and both processes optimised to achieve improved properties of the compound coatings. Metallographic, X-ray, wear, and corrosion test results for the compound coatings are presented.

Tribomechanical properties of cobalt-silicon nitride composite coatings

Abstract: Cobalt-silicon nitride composites were produced on mild steel by conventional electrocodeposition from a sulphate bath. These deposits showed better performance than pure cobalt in friction and wear tests against a hardened steel disc under dry sliding conditions. Changes of load and sliding velocity at the interface were also found to influence the tribological performance.

Influence of contaminants on gas nitriding

Abstract: Machining of steels not only affects the shape of the workpiece, but also influences surface structure and composition. At elevated temperatures and pressures during machining or the subsequent cleaning process cooling lubricants, hydraulic oils, machine greases, and cleaning residues can form stable adsorption or reaction layers. These layers can work as barriers and therefore inhibit surface modification reactions, as can be seen in the example of gas nitriding. The aim of the present investigation was to explore possible mechanisms of reaction inhibition and develop a nitriding process more reproducible in terms of reaction temperature and residence time. Samples prepared with residues of cleaning and machining processes on different alloy surfaces were nitrided under normal conditions and with extremely short residence times (1 < t < 20 min), in order to detect the effect of reaction and barrier layers on nitrogen acceptance during the initial stages of the nitriding process. For all alloys ex...

Fe–Cr–Ni–Mo–Ru ALLOYS PRODUCED BY LASER SURFACE ALLOYING

Abstract: Laser surface alloying of duplex 2205 stainless steel with prep laced ruthenium powder has been investigated. The microstructure and corrosion behaviour of the laser surface alloyed layers were examined by scanning electron microscopy, energy dispersive X-ray spectrometry, microhardness, and open circuit corrosion potential measurements. The microstructure of the surface alloyed layers consisted mainly of fine cellular dendrites; spectrometric analyses indicated that ruthenium contents as high as 23·30 wt-% could be produced. These features are presumed to be responsible for the higher hardness values observed in the alloyed layer. The open circuit corrosion potentials of the layers containing ruthenium were displaced towards more noble values in reducing acid solutions, resulting in spontaneous passivation of the alloy layer surface.

Quantitative characterisation of thin film structures

Abstract: The properties of thin films are functions of film structure and hence of production route. The structure of thin films consists of macro- and microscale features such as grains, together with pits, cracks, and inclusions, all of which are characterised by their spatial distribution, size, and shape. Some basic definitions and methods that can be used to characterise quantitatively the microstructures of thin films are presented. Examples are given for TiN films produced by plasma assisted chemical vapour deposition and physical vapour deposition.

Investigation of surface segregation on cold rolled steel by glow discharge spectrometry

Abstract: The composition of the uppermost surface layer of steel influences the coating properties. Surface segregation is generally considered to be one of the reasons for difficulties that can occur during hot dip galvanising. In the present work the surface composition of cold rolled, batch annealed steels and the change of composition after pickling, polishing, and shot blasting were studied using glow discharge optical emission spectrometry (GDOES) to explain the variations in the reactivity of the steels when undergoing hot dip galvanising. Surface segregation of all the elements included in the measurements (carbon, silicon, manganese, phosphorus, sulphur, chromium, nickel, aluminium, copper, and calcium) was observed. However, only silicon and phosphorus segregation clearly influenced the reactivity of the steel. Steel reactivity during galvanising could not be explained by the segregation of the outermost layer of the steel surface. Some elements (silicon, manganese, and aluminium) can have a rela...

Electric arc deposition of carbon steel coatings with improved mechanical properties

Abstract: Carbon steel coatings can be employed for hardfacing mechanical components used in moderately aggressive wear environments, provided they are sufficiently hardened by retained carbon However, when thermal spraying using compressed air as the atomising gas, properties of carbon steel coatings are degraded by carbon loss through oxidation and oxide inclusions It is shown that the physical and mechanical properties of these coatings can be greatly improved by spraying with inexpensive nitrogen produced on site by pressure swing adsorption or membrane separation, without the cost penalties incurred when using pure nitrogen In some circumstances, such coatings can be used to replace more expensive alloy hardfacing layers

FRICTION AND CORROSION RESISTANCE OF SPUlTER DEPOSITED SUPERSATURATED METASTABLE AI-Mo ALLOYS

Abstract: Two closed field unbalanced magnetrons with aluminium and molybdenum targets were used for the codeposition of Al-Mo coatings with different compositions. A pin on disc machine and a computer controlled potentiostat were used to evaluate the tribological and corrosion properties of the deposited alloys respectively. Results show that introducing molybdenum into aluminium coatings improves their poor tribological properties; Al-Mo coatings with different compositions showed low wear behaviour and for coatings with high molybdenum contents (> 80% ) friction coefficients against steel as low as 0·18 were obtained. The addition of molybdenum to aluminium coatings reduced their tendency to corrode and corrosion current density in a marine environment.