Showing papers in "Surface & Coatings Technology in 1994"

Protective coatings in the gas turbine engine

Abstract: The gas turbine provides one of the hardest environments challenging material systems today. Engine components are subject to rigorous mechanical loading conditions, high temperatures, and corrosive or erosive media. Historically, engine manufacturers have applied protective coatings to increase the durability and field performance of alloys or composites. Field experience as well as the development of processing technologies has led to the evolution of many new and improved coatings systems for a variety of engine component applications. In the fan and compressor areas, erosion-resistant coatings are used to minimize blade wear, antigallant coatings are used for fretting wear of blade root attachments and corrosion-resistant coatings are used for blade and disk materials. In the case of the combustor and turbine areas, diffusion and overlay coatings have been developed for oxidation and/or corrosion resistance and ceramic coatings are used widely as thermal barriers. In this paper, we shall review the current state of the art for coating technology in gas turbine engine applications with a look towards future needs and directions.

Metastable phase formation and enhanced diffusion in f.c.c. alloys under high dose, high flux nitrogen implantation at high and low ion energies

Abstract: The use of elevated target temperatures near 400 °C during high flux ion implantation of N2+ at energies ranging from 60 keV to 0.4 keV leads to a metastable, f.c.c. high nitrogen solid solution phase induced in austenitic (f.c.c.) Cr-containing stainless steels. This phase has not been produced in an f.c.c. Ni-Fe alloy containing no Cr. Penetration depths of the N are significantly larger than expected on the basis of known diffusion coefficients of N in Cr-containing stainless steels or in pure f.c.c.-Fe. X-ray diffraction data suggest unusual differences in N penetration and concentration depending on the f.c.c. grain orientation. Consideration is given to possible residual stresses induced by N expansion of the lattice and to the anisotropic elastic constants for austenitic stainless steels. The amount of N in interstitial solid solution approaches 40 at.% under the lower energy, higher flux conditions, but only in the (200) crystallographic planes parallel to the surface. The N-expanded f.c.c. phase with the highest amount of N is found by conversion electron Mossbauer spectroscopy to be magnetic in AISI 304 and 310 stainless steels. A comparison of our results with those from related methods such as conventional plasma ion nitriding and pulsed plasma ion implantation is made to demonstrate that the observed metastable solid solution phase is often produced by other methods provided that appropriate temperatures and processing times are used. The similar penetration depths of N in the f.c.c. Cr-containing stainless steels for the various methods are consistent with thermal diffusion and metastable solubility that are enhanced by the Cr. The metastability is associated with the low Cr mobility below about 450 °C and the strong N-Cr bond. Evidence for vacancy-enhanced diffusion is not found and we see at present no advantage to using ion energies higher than about 2 keV at these elevated temperatures for producing surfaces with optimized tribological behavior based on the extremely high strength, metastable, N solid solution phase.

Review of CVD TiN coatings for wear-resistant applications: deposition processes, properties and performance*

Abstract: Titanium nitride (TiN) deposited by chemical vapor deposition (CVD) techniques has been the premier wear-resistant coating in many applications for the last several decades. This review presents a history of developments in the technology of CVD TiN coatings. The CVD processes and equipment are discussed and the current knowledge on the thermodynamics and kinetics of the deposition process is presented. Various properties of CVD TiN coatings, such as hardness, adhesion, morphology and frictional properties, are discussed in the context of applications where wear resistance is required. Particular emphasis is given to applications in metal cutting where TiN has remained the mainstay of the various new and recent developments in coating design for cutting tools. Recent results of correlation between machining performance, deposition parameters, coating morphology and properties are presented. Throughout the paper, gaps in the existing understanding of the technology are pointed out. Finally, the paper presents a glimpse into the future of CVD TiN technology for wear-resistant applications.

Tribochemistry of diamond-like carbon coatings in various environments

Abstract: Diamond-like carbon films deposited on silicon wafers by r.f.-plasma-assisted chemical vapour deposition were friction tested in controlled atmospheres in a reciprocating pin-on-plate configuration using a steel sphere. Friction experiments were carried out in a vacuum range from 10 -7 to 50 Pa, in dry nitrogen and in ambient air. Analytical investigations of the wear process were peformed using transmission electron microscopy-electron energy loss spectroscopy and secondary ion mass spectroscopy. In all cases a transfer film was observed to form on the steel pin during the first 100 cycles, associated with relatively high values of the friction coefficient (0.2–0.3) at this stage. Beyond N =100 cycles the friction coefficient decreased to 0.006–0.008 in a vacuum below 10 -1 Pa and to 0.01–0.07 in a vacuum of 10–50 Pa and in dry nitrogen. The shearing ability of the interfacial film depends strongly on the nature of the atmosphere during friction, which affects the surface composition of the sliding counterfaces. A high vacuum is associated with ultralow friction and low wear. A poor vacuum and an inert atmosphere are associated with low friction and moderate wear. Ambient air is associated with relatively high friction and severe wear, coupled with the formation of roll-shaped debris of amorphous carbon containing iron oxide precipitates.

Improvement of MCrAlY coatings by addition of rhenium

Abstract: In order to improve the efficiency and power output of gas turbines, turbine inlet temperatures are being rapidly increased. Since high strength blade materials and sophisticated processes such as single-crystal solidification are nowadays available also for large parts, it is possible to increase metal surface temperatures to save cooling air. The increase in surface temperature causes a more severe oxidation attack on the blade coating, which is usually of the MCrAlY type in stationary gas turbines. Additionally, coating degradation by interdiffusion with the substrate and by thermal mechanical stress is increased. Considerable research has been carried out to improve the high temperature properties of MCrAlY (with M≡Co, Ni or combinations thereof) coatings by additions of minor alloying elements such as Si, Hf, Ta, Zr, etc. However, up to this time no work has been published on the properties of MCrAlY coatings with rhenium additions for stationary gas turbines. In this paper we report on the properties of MCrAlY coatings containing 1.5–10 wt.% Re. The coatings were applied to INCO 738 LC material by low pressure plasma spraying. Static and cyclic oxidation tests were carried out for up to 5000 h and 1500 cycles at 950 and 1000°C. Thermal fatigue tests under near-service stresses for up to 3000 cycles were performed. The microstructural stability and the interdiffusion behaviour were studied after long-time exposure. Rhenium additions can considerably improve the oxidation resistance of “classical” MCrAlY systems. Even more important, however, is the improvement in thermal cycle fatigue as properties as reflected by the number of cycles until crack initiation. Thus several of these coatings can be recommended for high temperature-high thermal load applications.

Structure and properties of Ti-B-N coatings

Abstract: Crystal structure studies on Ti-B-N coatings of various composition show a good agreement with the phase diagram predictions of Novotny and coworkers (1961) Simple considerations lead us to believe that the performance of such coatings can be optimized if they do not contain a soft phase such as Ti or h-BN and if the composition is located in a zone where the phases TiB2 and TiN coexist in nearly equal parts The microstructure plays an important role, in particular if the grain size of the coatings is in the nanometre range for Ti-B-N coatings In this case an inverse Hall-Petch effect leading to a softening of the coating might be observed Several preparation techniques for the synthesis of suitably composed coatings are presented and discussed These are, in particular: (a) Ti+ implantation into hexagonal BN layers; (b) co-sputter deposition from Ti and BN targets; (c) sputtering from a heterogeneously composed Ti-BN target; and (d) Ti/BN multilayer coating interdiffusion In spite of ultrahigh hardness and good adhesion to metallic substrates, most tribological tests revealed slightly worse performance than TiN and Ti(Al,N) coatings

Effect of vacuum arc deposition parameters on the properties of amorphous carbon thin films

Abstract: Hard and smooth films of amorphous carbon with thicknesses in the nanometer to micrometer range were formed on silicon substrates using a vacuum arc deposition technique In this technique, a carbon plasma is generated by a vacuum arc plasma source coupled with a magnetic filter for obtaining macroparticle-free amorphous carbon films The influence of the substrate bias voltage and pulsed bias duty cycle on the film properties was investigated A significant enhancement of the film quality and adhesion was achieved by applying a negative pulsed bias voltage to the substrate Nanoindentation, pin-on-disk tribotesting, surface profilometry, Rutherford backscattering spectroscopy, elastic recoil spectroscopy, and Raman spectroscopy were used to characterize the properties and structure of the amorphous carbon films It was found that the hardest films with the highest density and lowest friction coefficient were obtained at - 100 V pulsed bias voltage, whereas higher pulsed bias voltages improved the film adhesion and reduced the internal stress For -100 V pulsed bias voltage, the maximum film hardness was achieved with a 50% duty cycle, and was significantly higher than that produced with a dc bias

Superlubricity of MoS2: crystal orientation mechanisms

Abstract: We have investigated the origin of the extraordinary low friction coefficient (in the 10 -3 range or even less) of pure and stoichiometric sputtered MoS 2 coatings, in ultrahigh vacuum. In these conditions, shear strengths of the interface as low as 1 MPa were measured. Importantly, the tribometer was operating in macroscopic contact conditions, typically at a long time-length scale. Friction-induced orientation of (0001) basal planes of MoS 2 grains parallel to the sliding direction was first verified by means of electron diffraction. Friction-induced rotation of these crystals around the c axis, during intercrystallite slip in the contact, was investigated by high resolution transmission electron microscopy performed on selected wear fragments. Atomic force microscopy at atomic resolution was also carried out on the surface inside and outside the wear scar. The data indicated that the vanishing of the friction force was due to frictional anisotropy in the interface between nanometre-scale domains in rotational disorder (intercrystallite slippage of incommensurate sulphur-rich hexagonal lattices). The term superlubricity was used here because of the zero friction state that could be theoretically predicted in these conditions. Finally, the mechanisms of MoS 2 superlubricity are thought to depend on the proper combination of the grain size, the two crystal orientation effects and the absence of contaminants.

Role of phosphorus in the electrochemical behavior of electroless Ni-P alloys in 3.5 wt.% NaCl solutions

Abstract: Electroless Ni-P deposits with phosphorus contents ranging from 4.8 to 12.8 wt.% were examined using d.c. polarization curves and a.c. electrochemical impedance spectroscopy techniques to characterize the effect of phosphorus on the corrosion behavior of electroless nickel-plated mild steel in neutral and alkaline 3.5 wt.% NaCl solutions. The anodic polarization behavior of the Ni-P alloys in NaCl electrolyte was very similar for all deposits except Ni-4.8P. According to the electrochemical impedance data, the charge transfer resistance R ct and film resistance R f of these deposits have subsequently been studied as functions of the phosphorus content. It was shown that the R ct and R f values increased with increasing phosphorus content of the electroless nickel coating. The Ni-P deposits remained bright in appearance during the periods of electrochemical experiment, and no evidence of pitting was observed after testing. Based on X-ray photoelectron spectroscopy analysis, the passivation of Ni-P alloys was attributed to the formation and adsorption of a layer of hypophosphite.

Thick Ti/TiN multilayered coatings for abrasive and erosive wear resistance

Abstract: Plasma-assisted physical vapour deposition (PAPVD) of hard coatings such as TiN is widely employed on cutting and forming tools where a thin coating ( i.e. 1–5 μm) is sufficient to control the primary (usually “adhesive”) wear mechanism. Diversification into other application areas, such as components for which abrasive wear resistance is the main requirement, has however been constrained by coating thickness limitations (typically to below 10 μm)—due to debonding caused by the build-up of internal stresses. The hardness and “load-support” mismatch between such coatings and the more widely used metallic ( i.e. non-tool steel) substrate materials (both ferrous and non-ferrous) also leads to a requirement to modify coating-substrate interfacial properties and decouple substrate mechanical (and-to some extent-electrochemical) effects from the important coating surface or “near-surface” requirements. Here we report a technique for interlayering of TiN with titanium to produce a “toughened” composite coating which can be greater than 60 μm thick. By control of layer thickness and interfacial properties, the brittle-fracture failure regime normally associated with TiN on relatively soft substrates can be replaced with a ductile “tearing” mechanism which provides improved performance in three-body abrasive wear tests and airborne-particulate erosion simulation trials. The implications of multilayered and multitreatment coatings in widening the applicability of PAPVD treatments are also discussed.

The properties of TiN films deposited by filtered arc evaporation

Abstract: Titanium nitride films were deposited on steel and silicon substrates using a filtered arc evaporation process. The microhardness, crystallite size, residual stress, adhesion, surface roughness and crystallographic orientation of the films were studied as a function of substrate bias over the range 0 to -400 V. The mechanical and structural properties were found to depend upon the degree of substrate bias. The spectral reflectance in the region between 250 and 2500 nm was measured and compared with Kr + ion-assisted deposited TiN films. In addition, the electrical resistivity and the superconducting transition temperature of the filtered arc deposited TiN films were measured to be 20 μΩ cm and 4.3 K respectively.

Microstructural investigation of droplets in arc-evaporated TiN films

Abstract: The microstructure and composition of macro particles (droplets) in TiN films deposited by arc evaporation on cemented-carbide substrates were investigated using a combination of scanning electron microscopy, cross-sectional transmission electron microscopy (TEM) including lattice resolution TEM, X-ray diffraction (XRD), energy-dispersive X-ray analysis and electron energy loss spectroscopy. The apparent surface number density of droplets, with diameters of 0.1–10 μm, was found to be about 10 7 cm −2 . Droplets were incorporated in the film at various distances from the substrate surface. In between the droplets, the TiN films exhibited a dense columnar microstructure. On top of the incorporated droplets, the TiN films grew in a pronounced columnar structure with a column diameter close to the droplet diameter. The core of the droplets consisted of equiaxed grains of an α-Ti superstructure containing approximately 3–5 at.% N whereas the rim of the droplets had increasing nitrogen content up to 50 at.% XRD showed evidence for the presence of Ti 2 N possibly at the rim of the droplets. Beneath each droplet a large voided region was observed with the shape of a flattened torus as a consequence of droplets being incorporated in the solid state and subsequent shadowing of the Ti flux to the film.

High-current arc-a new source for high-rate deposition

Abstract: A new type of arc evaporator, the so-called HCA (high-current pulsed arc), was developed to increase the deposition rate by several times in comparison with conventional arc evaporation units. This device makes use of pulsed arc discharges with nearly sinusoidal current shapes (duration 0.1–1 ms, peak current > 5 kA), which are ignited repeatedly (0–300 s-1) in the centre of a circular cathode. The high pulse current drives the cathode spots radially at high velocity, but before touching the cathode border the arc is switched off and another cycle is started in the centre. The new arc evaporator can work with an average arc current as high as 1000 A. This represents an increase of more than five times over the d.c. currents of 100–200 A usually used. The controllable arc duration and repetition frequency mean that only a well defined circular area of erosion is affected by the arc evaporation. Besides the high rate, there are some other advantages: the deposition rate can easily be controlled and adjusted in a range from zero to a maximum value. The pulsed arc operates with peak deposition rates about 20 times higher than an ordinary arc evaporation unit. Using this method a modified film structure and/or composition can be expected. The number of droplets will be strongly diminished. Using two or more evaporation units, films with a well defined modulated composition and structure (e.g. multilayer films with single film densities of some nanometres) can be produced effectively and with high reproducibility. All the advantages of vacuum arc deposition (high degree of ionisation, ion energies of about 20–50 eV, option for reactive deposition) are enhanced or preserved.

TiCr)N coatings deposited by cathodic vacuum arc evaporation

Abstract: (TiCr)N coatings were deposited using two cathodic vacuum arc evaporators fitted with chromium and titanium cathodes and with an alloyed cathode. Structural investigations were carried out: phases, textures, crystallite sizes and lattice parameters. The influences of the bias voltage and nitrogen pressure on the coatings are characterized. Hardness measurements and scratch test results are presented.

Plasma-assisted nitriding of aluminum

Abstract: Three aluminium materials, Al 99.5, AlMgSi 0.5 and AlZnMgCu 1.5 (1050, 6063 and 7075), were nitrited in a modified plasma- nitriding unit with a pulsating d.c. power supply. Prior to nitriding, the samples were cleaned in a plasma-assisted sputtering process in gas mixtures of argon and hydrogen. Shiny black aluminum nitride (AIN) layers with hexagonal crystal structure were formed on top of all three substrate materials, but distortion, microcracks and delamination of the AIN layers were observed too. Therefore AIN layers with high hardness and good adhesion could be obtained only by a proper adjustment of the nitriding parameters, especially the nitriding time and temperature, to keep the thickness of the AIN layer below about 3 μm. The wear resistance of the nitrided specimens was significantly increased compared with those without nitridingtreatment. On the other hand, a significant increase in corrosion resistance of the AIN layer in 1N HCl could be achieved only by using nitrided Al 99.5 specimens.

High dislocation density structures and hardening produced by high fluency pulsed-ion-beam implantation

Abstract: The paper presents a review of experimental data on the “long-range effect” (a change in dislocation structure and in physicomechanical properties at distances considerably greater than the ion range value in ion-implanted metallic materials and semiconductors). Our results of electron microscopy studies of high density dislocation structure in ion-implanted metallic materials with different initial states are given. It has been shown that the nature of the dislocation structure and its quantitative characteristics in the implanted metals and alloys depend on the target initial state, the ion type and energy and the retained dose. The data obtained by different workers are in good agreement both with our results and with each other as well as with the results of investigation of macroscopic characteristics (wear resistance and microhardness). It has been established that the “long-range effect” occurs in metallic materials with a low yield point or high plasticity level and with little dislocation density in their initial state prior to ion implantation.

Oxide formation of K38G superalloy and its sputtered micrograined coating

Abstract: The oxide scales formed on the cast Ni-based alloy K38G and its sputtered, micrograined coating at 1000°C in air have been investigated by optical, electron probe microanalysis, scanning and transmission electron microscopy, and X-ray diffraction measurements. Complex oxide scales formed on the cast alloy composed of the outer layer of Cr 2 O 3 , TiO 2 and NiCr 2 O 4 and the inner layer of Al 2 O 3 are prone to spallation. On the contrary, only a thin oxide layer of Al 2 O 3 can be observed on the sputtered alloy, and it was very adherent. The formation of the Al 2 O 3 layer and its excellent adhesion may be attributed to the microcrystalline grains of the sputtered material.

Formation of hydroxyapatite coating on pure titanium substrates by ion beam dynamic mixing

Abstract: A hydroxyapatite (HAp) coating was deposited onto pure Ti substrates using an ion beam dynamic mixing (IBDM) method. In this study, a high energy Ca + ion beam of 50 keV was irradiated concurrently during evaporation of HAp vapor onto Ti substrates to deposit the HAp film. The structure, solubility in Hanks' solution and adhesive strength of the coatings were evaluated. The as-coated film was amorphous and was almost dissolved within 1 day in the solution. A thin Ca-P-Ti hydroxide complex layer, however, was formed on the substrate surface, indicating that the IBDM film might possess a “self-repairing function.” The crystallinity of the HAp film was improved by heat treatment at 873 K in air, and the film obtained was slightly soluble in the solution. The adhesive strength of the as-coated film was more than 59 MPa. The strength decreased to 41 MPa after the heat treatment, but even this value exceeds that obtained using conventional methods.

Concept of microsmoothing in the electropolishing process

Abstract: Electropolishing of metals and numerous alloys can provide smooth, bright and reflective surfaces that exhibit superior corrosion resistance compared with untreated materials. These features are achieved mainly after electropolishing pure metals and single-phase alloys. Earlier researchers have indicated that, in comparison with other treatments (mechanical polishing, chemical etching), electropolishing produces a much thinner surface layer with changed and specific properties. This implies a difference in the solid surface chemistry which is valuable from the viewpoint of the technical usefulness of such surfaces. Experimental investigations carried out on various metals and alloys have proved that both electrochemical and hydrodynamic conditions are responsible for the positive results obtained in the process. The aim of the present work is to provide a new concept of electropolishing covering transport phenomena such as diffusion and adsorption as well as the hydrodynamic conditions.

Growth of thin films with preferential crystallographic orientation by ion bombardment during deposition

Abstract: Ion bombardment during the growth of a film may considerably influence the structure of this film. Often a preferred crystallographic orientation of the film grains is observed. As an example, NaCl-type crystals tend to grow in a preferred [111] direction normal to the film plane owing to surface energy effects in connection with close-packed structures. However, upon ion irradiation the alignment shifts towards [100]. The ions channel along this direction deep into the lattice which as a consequence suffers from less radiation damage and sputtering. Both effects are in competition with each other during film growth. The most important factor for preferential growth is the ratio of impinging ions to arriving atoms. When off-normal ion incidence is applied, the alignment of the crystals follows the ion beam line to a certain extent. When irradiation effects and surface energy effects are combined by off-normal bombardment at selected angles, an azimuthal order can be achieved in addition to the surface normal alignment. This may result in biaxially aligned films with the perspective of unique properties. The above-mentioned effects are discussed on the basis of results from X-ray diffraction measurements on titanium nitride films grown by metal evaporation under nitrogen or argon ion bombardment.

Superhard coatings for metal cutting applications

Abstract: An effective metal cutting tool is usually a combination of a hard coating and a tough substrate. The successful deposition of diamond outside its thermodynamic stability range has stimulated the development of a new class of cutting tools: those with diamond coated inserts at any desired style and edge geometry. The successful implementation of diamond coatings also expedited similar research in the deposition of cubic boron nitride. This paper presents superhard coated tools, with an emphasis on diamond coated WC-Co tools, the corresponding deposition technologies and the foreseen metal cutting applications.

The effect of roughness on the friction and wear of diamond thin films

Abstract: The friction and wear properties of polycrystalline diamond thin films depend on both the counterface material and the properties of the diamond layer. The roughness of the diamond is critical because it controls the amount of abrasive damage to the counterface. Reducing the surface roughness by polishing or control of the diamond growth process leads to reductions in both friction and the wear of the counterface. For low friction, the smoothest polished films or those which consist of microcrystalline diamond give the best performance and lead to only small amounts of counterface wear. When diamond slides against other materials a transfer layer forms which controls friction. Very little diamond debris is found in this layer for smooth films, but some debris from the fracture of the highest asperities is embedded in the transferred material for rougher films. There is no evidence for reaction between the diamond debris and the transferred material from a sapphire counterface.

Titanium boron nitride coatings of very high hardness

Abstract: Ti-B-N coatings of variable composition have been sputter deposited from a heterogeneously composed Ti-BN target, consisting of a boron nitride base plate on which small Ti platelets were regularly arranged. By varying the number of platelets the concentration ratio c Ti / c B of the target and therefore also of the coating could easily be varied. Inhomogeneities in the chemical composition were in the per cent range. Very hard coatings of up to 55 GPa were obtained by sputtering with a substrate bias of - 150V at a substrate temperature of 400°C. The hardness maximum was found at a chemical composition where both the TiB 2 and the TiN phases coexist in equal concentrations. All coatings show very broad diffraction peaks indicating average grain sizes in the nanometre range where the Hall-Petch relation is no more valid. With coatings composed of very small grains an inverse Hall-Petch effect was observed. The tribological performance of the Ti-B-N coatings is promising; however, in general it is worse than that observed with TiN coatings. The main reason seems to be the relatively low cohesive strength of the material.

Metallic interlayers between steel and diamond-like carbon

Abstract: Many processes for the deposition of diamond-like carbon (DLC) are carried out just above room temperature. The low process temperature makes it simple to coat a wide variety of materials but can lead to serious adhesion problems between the coating and the substrate. In this work DLC coatings on steel for tribological applications were developed. DLC was deposited on stainless steel (DIN 1.4305) by pulsed laser ablation of graphite. Generally the adhesion of DLC on steel was very poor. By applying different metallic interface layers (e.g. Cr, Mo, Si, Cu), the adhesion has been improved considerably. The microstructure of the interfaces has been studied by cross-sectional transmission electron microscopy, including high resolution imaging and electron diffraction, but no evidence for a crystalline reaction zone was observed. The adhesion of the coating has been tested by ultrasonic cavitation and the scratch test. A rank order of intermediate layer metal combinations was established. Generally the combination of Cu and a metal belonging to one of the IVb, Vb or VIb groups of elements was observed to give good adhesion. Cu/Cr appeared to give the best adhesion.

Composition, structure and tribological properties of amorphous carbon nitride coatings

Abstract: Carbon nitride thin films were synthesized using d.c. unbalanced magnetron sputtering of a high-purity graphite target in a nitrogen-containing plasma onto various substrates held at ambient temperatures. The N/C ratio was found to vary from zero to 0.8 depending on deposition conditions. There was evidence of multiple bonding states for carbon and nitrogen (sp, sp2 and sp3), and the bulk of the film was found to be amorphous. Nanoindentation studies showed that under appropriate substrate bias conditions the carbon nitride coating hardness can be enhanced to levels well above that of conventional amorphous hard carbon coatings. CNx coatings replicate the substrate topography, giving rise to surface roughness equal to or better than the original substrates. Lubricated tribotesting showed the relationship between wear performance and deposition conditions. Under pure sliding in air, the CNx coating provided a 30-fold reduction in wear rate for M2 steels (compared with uncoated M2) under our testing conditions.

Oxidation behaviour of chromium-based nitride coatings

Abstract: Chromium-based nitride coatings (Cr2N, (Cr,Pd)N, (Cr,Al)N, (Cr,Al,Pd)N) were deposited on high-speed steel substrates by reactive magnetron sputtering. Annealing of the coatings was performed in air over the temperature range between 300 and 800°C. The annealing time was varied from 5 min up to 4 h. The oxidized coatings were analyzed using AES sputter depth profiling. The Cr2N coatings show a diffusion-controlled formation of Cr2O3 layer with an activation energy of 1.27 eV (outward diffusion of Cr). The (Cr,Pd)N coatings show, besides a Pd enrichment below the Cr2O3 layer, a very similar oxidation behaviour with a slightly lower activation energy of 1.16 eV as compared with Cr2N. In the case of (Cr,Al)N and (Cr,Al,Pd)N a sequence of different oxide layers (Cr-oxide, Al/Cr-oxide, oxynitride) is built up owing to complicated diffusion processes (outward diffusion of Cr and Al, inward diffusion of O). In (Cr,Al)N a wide oxygen interdiffusion region is observed, whereas in (Cr,Al,Pd)N this region is not very large owing to the small oxygen affinity of pd. In both the oxidation processes of (Cr,Al)N and (Cr,Al,Pd)N, the diffusion of chromium is identified as the rate-limiting process, with activation energies of 0.62 eV for (Cr,Al)N and 0.77 eV for (Cr,Al,Pd)N.

Mechanical and tribological characterization of low-temperature deposited PVD TiN coatings

Abstract: Three potential processes for TiN deposition at low temperatures (high current density plasma beam evaporation (200–320 °C), high current density plasma beam activated electron beam evaporation (220°C) and high current density plasma beam assisted sputtering (220°C)) are evaluated with respect to the resulting surface topography, hardness, scratch resistance and abrasive wear resistance of the deposited coatings. TiN coatings deposited using three different processes operating at standard temperature (450°C) were used as references. It is shown that it is possible to obtain low-temperature TiN coatings with retained properties (hardness, scratch and abrasion resistance) as compared with TiN coatings produced at standard temperatures. The results also indicate that, at low temperatures, activated evaporation seems to yield coatings with better mechanical and tribological properties than sputtering. An important observation is that the performance ranking of the investigated coatings differs with the parameters measured. This, in turn, implies that several methods must be utilized for evaluation of a candidate coating-substrate composite for a given application.

A corrosion study of TiN (physical vapour deposition) hard coatings deposited on various substrates

Abstract: Titanium nitride (TiN) coatings 2.8 υm thick, reactively deposited on various substrates by physical vapour deposition, were studied using electrochemical methods in combination with scanning electron microscopy and Auger electron spectroscopy. Three different substrates were used, namely stainless steel AISI 304, high speed ASP steel ASP 30 and stabilized iron CK 45. Multisweep cyclic voltammetry experiments in acetate buffer solution were performed in order to test the microporosity of the coatings. An Auger depth analysis performed on TiN coatings polarized at potentials more positive than 1.3 V in 0.5 M H2SO4 proves the formation of an oxidized layer, indicating the partial transformation of titanium nitride to a titanium oxynitride layer. A change in the corrosion behaviour associated with the presence of the TiN coating was observed in 0.5 M NaCl. Whereas the uncoated iron undergoes general corrosion, localized corrosion occurs on the TiN-coated iron. This localized corrosion is a consequence of the microdefects present in the coating.

Modelling the hardness response of coated systems: the plate bending approach

Abstract: In order to design the hard coating best suited to engineer the surface of a given substrate for a particular application, it is important that we have a clear picture of the mechanisms by which thin hard coated systems work. In this paper we describe a quantitative model designed to explore the increase in systems' hardness attributable to stresses created on the coating by the upthrust of substrate. material flowing plastically in response to indentation. Thus the model considers how the upwelling of displaced substrate material creates an upward pressure on the coating inducing elastic flexure which, in turn, resists the substrate deformation. The influence of this flexure has been investigated using a combination of existing analytical solutions (e.g. plate theory) to determine the overall effect on the indentation response of coated systems. The resulting model is critically compared with previous work. Experimental verification is provided by diverse experimental data from indentation tests on thin (1–24 μm) coatings of TiN and Cr on various metal substrates, sugar on chocolate substrates and other results drawn from the literature. The model helps to identify Eh 3 / d 3 H 0 as a dimensionless parameter which provides a means of condensing all available data onto a single “master” curve. Further developments are discussed.

Role of Y during high temperature oxidation of an M-Cr-Al-Y coating on an Ni-base superalloy

Abstract: A study was carried out to determine the role of Y during high temperature oxidation or an M-Cr-Al-Y-type coating on an Ni-base superalloy using various techniques of electron microscopy. Prior to oxidation exposures, Y was found to segregate within an external surface layer of the coating about 5 μm in thickness as well as within the interdiffusion zone. After exposure at 1000 or 1100°C in still air, Y exhibited a preferential tendency for segregation at grain boundaries of α-Al2O3 scale developed by the coating, however, a finite concentration of Y was also present in solid solution. In addition, some of the Y was found to oxidize into a Y-rich oxide “pegs” within the α-Al2O3 scale and extending into the coating. Experimental results showed that segregation of Y to grain boundaries could maintain a fine-grained scale after exposure for up to 1000 h at 1000 or 1100°C. It was concluded that, for the coating-substrate system studied, Y could improve the protective nature of α-Al2O3 scale by various mechanisms operating simultaneously.