Showing papers in "Surface & Coatings Technology in 1998"

Progress in coatings for gas turbine airfoils

Abstract: The development of ever more efficient gas turbines has always been paced by the results of research and development in the concurrent fields of design and materials technology. Improved structural design and airfoil cooling technology applied to higher strength-at-temperature alloys cast by increasingly complex methods, and coated with steadily improved coating systems, have led to remarkably efficient turbine engines for aircraft propulsion and power generation. For first stage turbine blades, nickel-based superalloys in various wrought and cast forms, and augmented by coatings since the 1960s, have been singularly successful materials systems for the past 50 years—and still no real world substitutes are on the horizon. This paper traces the history of protective coatings for superalloy airfoils beginning with the simple aluminides, followed by modifications with silicon, chromium and platinum, then MCrAlY overlay coatings, and finally the elegant electron beam vapor deposited ceramic thermal barrier coatings recently introduced to service. The publicly available results of several decades of research supporting these advances are highlighted. These include generic research on oxidation and hot corrosion mechanisms of superalloys and coatings, the intricacies of protective oxide adherence, mechanisms of low temperature (Type II) hot corrosion, and of aluminide coating formation and mechanical properties of alloy–coating systems. With no promising turbine materials beyond coated nickel-base superalloys apparent in the foreseeable future, continued progress will likely be made by further refinement of control of thermally grown oxide adherence, and by more cost effective manufacturing processes for contemporary types of protective coatings.

On the hardness of coated systems

Abstract: The hardness of a number of coated systems has been measured using a variety of experimental techniques ranging from traditional macro-Vickers indentation to ultra-low-load depth-sensing nanoindentation. This has allowed the hardness response to be measured over scales ranging from those less than the coating thickness, where a coating-dominated response is expected, to much more macroscopic scales where system behaviour is dominated by the substrate. The objective has been to construct a mathematical description of the hardness performance of coated systems which well describes the behaviour over this wide range of scales. Previous attempts at such quantitative descriptions have usually involved models focusing on some particular deformation mechanism (e.g. plasticity, elastic response or fracture). In contrast, this paper presents a new approach to analysing hardness data essentially using dimensionless parameters containing descriptors equally applicable to either plasticity- or fracture-dominated behaviour with all scales measured relative to the coating thickness. The model shows an excellent fit to a wide range of experimental data. Furthermore, once the fit has been made, not only can some deductions be made regarding dominant deformation mechanisms, but the model allows predictions of the contact response of other coated systems to be made.

Recent progress on the tribology of doped diamond-like and carbon alloy coatings: a review

Abstract: Diamond-like carbon (DLC) coatings have been widely recognized as being a wear-resistant solid lubricant with a low friction coefficient. Its tribological behavior strongly depends both on the tribotesting conditions and the nature of the coating, which in turn depends on the technique used for film deposition. Recently, there have been several attempts to improve the tribological behavior of DLC coatings by the addition of elements, such as silicon, nitrogen, fluorine and various metals. The paper will present an updated review of the tribological properties of doped DLC, in comparison with the conventional hydrogenated and non-hydrogenated carbonaceous films.

Improving the properties of titanium nitride by incorporation of silicon

Abstract: Thin films of Ti–Si–N have been deposited by physical vapor deposition (PVD) with the intention to improve the wear resistance of TiN coatings. The coatings are prepared by reactive unbalanced magnetron sputtering using two separate Ti and Si targets and a rotating substrate holder. The silicon concentration in the deposited films varies between 0 and 15 at.%. SEM observations and X-ray diffraction analysis (XRD) show that the addition of Si to TiN coatings transforms the [111] oriented columnar structure into a dense finely grained structure. From TEM investigations and XRD analyses, the crystallite sizes of TiN are observed to be below 20 nm. XPS analysis shows the presence of silicon nitride, while electron and X-ray diffraction results do not suggest the presence of crystalline Si3N4. This result clearly indicates that these films have a composite structure consisting of TiN nanocrystallites embedded in amorphous silicon nitride. The hardness of the nc-TiN/a-SiNx coatings reaches 3500 HV0.1. The abrasion resistance measured by ball cratering can be enhanced by a factor of 6 in comparison with TiN deposited under the same conditions.

Plasma treatments and plasma deposition of polymers for biomedical applications

Abstract: The aim of this review is to describe how biomaterial engineering processes, particularly in the field of polymer modifications for enhanced blood-compatibility, can take advantage of low-pressure plasma techniques. It will also be emphasized how a diagnostic approach on both plasma and surface reactions can lead to understand and control the chemistry of plasma processes.

Work function of transparent conducting multicomponent oxide thin films prepared by magnetron sputtering

Abstract: The work function of transparent conducting multicomponent oxide (TCO) films is reported. TCO films consisting of binary oxides, such as In 2 O 3 , SnO 2 and ZnO, and ternary oxides, such as Zn 2 In 2 O 5 , In 4 Sn 3 O 12 , GaInO 3 , ZnSnO 3 and MgIn 2 O 4 , were prepared by magnetron sputtering. In addition, transparent conducting films consisting of multicomponent oxides composed of combinations of these binary or ternary oxides were also prepared by magnetron sputtering. The work function of these TCO films was measured by ultraviolet photoelectron spectroscopy operated in air. It was found that the work function as well as the electrical, optical and chemical properties of transparent conducting multicomponent oxide films could be controlled by varying the chemical composition.

Surface modification of titanium alloys for combined improvements in corrosion and wear resistance

Abstract: A simple and effective surface modification technique, namely palladium-treated thermal oxidation (PTO), has been developed in the present research. Comparative investigations on both corrosion and wear resistance have been carried out on surface-engineered titanium-based materials by conventional plasma nitriding (PN), thermal oxidation (TO), and the newly developed palladium-treated thermal oxidation (PTO). Both the TO- and PTO-treated materials have a significantly superior corrosion resistance in boiling HCl solutions compared to the PN-treated and untreated materials. The lifetime for the protective surface layer breakdown of the TO-treated titanium in boiling 20% HCl solution is about 13 times that of the PN-treated titanium, whereas the lifetime of the PTO-treated material has been increased further by a factor of 2.6 over the TO-treated material. The PTO-treated material has shown a better anti-scuffing capacity than the TO-treated material under oil-lubricated conditions. Characterisation of both the TO- and PTO-treated surface layers was performed using glow discharge spectrometry (GDS), X-ray diffraction (XRD) and scanning electron microscopy (SEM).

Recent progress in the superhard nanocrystalline composites : towards their industrialization and understanding of the origin of the superhardness

Abstract: Recent progress is presented regarding the development of plasma induced CVD technique for coatings of non-planar and thick tools and of the understanding or the origin of the high thermal stability of the nanocrystalline TiN/amorphous Si 3 N 4 superhard coatings up to 1100°C. A chlorine free, combined plasma CVD and PVD technique was developed for the deposition of superhard nc-TiN/c-BN nanocomposites. These coatings are compatible with the majority of engineering materials, such as ferrous and aluminum alloys and others. Their hardness reaches 70 GPa and elastic modulus E/(1-ν 2 )≈585 GPa. An industrial production reactor was built which enables one to deposit such coatings on cutting tools.

State of the art in hard coatings for carbide cutting tools

Abstract: The majority of carbide cutting tools in use today employ hard coatings because coatings offer proven benefits in terms of tool life and machining performance. Continuing development of the chemical vapor deposition (CVD) coating process, the most widely used technique, has produced complex multilayer coatings tailored for specific applications and workpiece materials. These coatings include alumina layers of different crystal structures, and TiCN layers applied by high- or moderate-temperature (MT-CVD) processes. Over the last decade, coatings applied by physical vapor deposition (PVD) have gained acceptance in applications requiring sharp edges or those featuring interrupted cuts. Originally limited to TiN coatings, the PVD offering now includes TiCN and TiA1N coatings which provide better high-speed performance and increased abrasive wear resistance. In the area of superhard coatings, improvements in deposition processes and coating adhesion have resulted in diamond-coated carbide tools that have begun to play an important role in machining non-ferrous and non-metallic materials. This paper presents the state of the art in hard coatings for carbide cutting tools including discussion of coating characteristics and applications.

Sintering and Creep Behavior of Plasma-Sprayed Zirconia and Hafnia Based Thermal Barrier Coatings

Abstract: The sintering and creep of plasma-sprayed ceramic thermal barrier coatings under high temperature conditions are complex phenomena. Changes in thermomechanical and thermophysical properties and in the stress response of these coating systems as a result of the sintering and creep processes are detrimental to coating thermal fatigue resistance and performance. In this paper, the sintering characteristics of ZrO2-8wt%y2O3, ZrO2-25wt%CeO2-2.5wt%Y2O3, ZrO2-6w%NiO- 9wt%Y2O3, ZrO2-6wt%Sc2O3-2wt%y2O3 and HfO2-27wt%y2O3 coating materials were investigated using dilatometry. It was found that the HfO2-Y2O3 and baseline ZrO2-Y2O3 exhibited the best sintering resistance, while the NiO-doped ZrO2-Y2O3 showed the highest shrinkage strain rates during the tests. Higher shrinkage strain rates of the coating materials were also observed when the specimens were tested in Ar+5%H2 as compared to in air. This phenomenon was attributed to an enhanced metal cation interstitial diffusion mechanism under the reducing conditions. It is proposed that increased chemical stability of coating materials will improve the material sintering resistance.

Relationships between residual stress, microstructure and mechanical properties of electron beam–physical vapor deposition thermal barrier coatings

Abstract: Residual stresses develop in coatings during deposition and can have a large impact on coating mechanical properties and durability. In this study, in-plane residual stresses in Electron beam–physical vapor deposited (EB–PVD) thermal barrier coatings (TBCs) were characterized by the change in substrate curvature upon coating removal, and in-plane elastic moduli were measured from the resonant frequency of the coating–substrate system. Variations in deposition conditions were observed to produce in PVD TBCs a wide range of stress levels, between −70 and 20 MPa. The residual stress was observed to be correlated strongly with the in-plane elastic modulus. A significant difference in the in-plane elastic modulus was measured along different directions of PVD TBC specimens fabricated by rotating the specimens over the evaporation source. The elastic modulus in the direction perpendicular to the axis of rotation was always significantly lower than the modulus measured along the axis of rotation. The elastic modulus measured perpendicular to the axis of rotation was associated with compliant microstructural features produced by the rotation of the substrate over the melt pool. Strain tolerance was measured directly by a new mechanical test that measured the strain at delamination of a coating from an edge-initiated crack from a substrate that was loaded in compression. The strain tolerance of the coating decreased with increasing residual stress.

Comparison of transparent conductive oxide thin films prepared by a.c. and d.c. reactive magnetron sputtering

Abstract: Antimony-doped tin oxide and aluminum-doped zinc oxide films have been prepared by reactive a.c. and d.c. magnetron sputtering (a.c. excitation at frequency of 40 kHz; twin-cathode arrangement) from metallic targets at substrate temperature of about 573 K. The optical, electrical and structural properties of the sputtered SnO2:Sb and ZnO:Al thin films of different dopant concentrations have been investigated by means of optical spectroscopy (UV-IR), X-ray diffraction. Hall mobility and conductivity measurements. For antimony-doped SnO2 films a minimum resistivity of 1.5 × 10−3 Ω cm at high transparency (larger than 88% at film thickness of 250 nm) has been observed at dopant concentrations of about 1.2 at.% Sb in the layers. Low resistivity of 4.0 × 10−4 Ω cm and transmission in the visible spectral range of about 89% at film thickness of 550 nm has been obtained for as prepared aluminum-doped ZnO thin films. Furthermore, the investigations on a.c. plasma discharges using a planar plasma probe analyzer have revealed higher ion energies (up to some tenths of eV) and about 10 times higher ion current densities as in the case of d.c. magnetron sputtering at nearly the same deposition conditions.

Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys

Abstract: The problem of technical and economical optimization of the process of micro-arc discharge oxidation of high-strength aluminium for the fabrication of oxide ceramic layers for tribotechnical purposes is considered in terms of experimental design. To estimate the effectiveness of the process, a generalized parameter is used which accounts for oxide mass yield as a principal parameter, and mechanical and geometrical characteristics of the layer as restricting parameters. The methods of chemical weight, scanning electron microscopy, optical and durometric analyses are used. The influence of the silicate–alkali electrolyte composition and the amount of electricity carried through the cell on the layer properties is discussed. The response surface of the generalized parameter is plotted with the aid of desirability functions. The area of regimes corresponding to 2–3 g l −1 KOH and 2–3 g l −1 Na 2 SiO 3 electrolyte composition and (2.50–3.33)×10 3 C m −2 of carried electricity is outlined for the most effective fabrication of uniform oxide layers with 165–190 μm thickness and 18–23 GPa hardness.

Formation of bilayer Ni–SiC composite coatings by electrodeposition

Abstract: Composite plating is a method of co-depositing fine particles of metallic, non-metallic compound or polymers in the plated layer to improve material properties, such as wear-resistance, lubrication, or corrosion resistance. Bilayer Ni–SiC composite coatings were produced in this research on the base of a reported study. Before producing bilayer Ni–SiC composite coatings, effects of the particle size, the particle content, the pH of the electrolyte, the temperature, the current density, the stirring rate on the amount of SiC deposited in the Ni layer were investigated. By manipulating the current density and the plating time properly, bilayer Ni–SiC composite coatings were produced and mechanical properties of these coatings were evaluated by micro-indentation hardness tests.

Mechanical and tribological properties of multilayered PVD TiN/CrN, TiN/MoN, TiN/NbN and TiN/TaN coatings on cemented carbide

Abstract: Four different multilayered coatings TiN/CrN, TiN/MoN, TiN/NbN and TiN/TaN, were deposited on cemented carbide A hybrid PVD process consisting of ion plating (TiN) and magnetron sputtering (CrN, MoN, NbN and TaN) was used The chemical and phase composition, morphology and microstructure of the coatings were assessed using auger electron spectroscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy, respectively In addition, all coatings were characterised with respect to their residual stress, hardness, cohesion/adhesion and abrasive wear resistance A PVD TiN coating was used as a reference throughout the investigation All coatings exhibited a cubic NaCl structure Furthermore, all multilayered coatings, except TiN/MoN, were found to have a strongly preferred (200) orientation TiN/MoN displayed a mixture consisting of (111), (200) and (220) orientations TiN displayed a (111)-preferred orientation It was found that the mechanical and tribological properties of the multilayered TiN/metal nitride coatings, in general, were superior to those of homogenous TiN The overall best performance was found for the TiN/CrN coating, whereas TiN/TaN showed the highest abrasive wear resistance

Effect of austenitic stainless steel composition on low-energy, high-flux, nitrogen ion beam processing

Abstract: A collection of 16 metals with the face-centered-cubic (fcc) crystal structure, including stainless steels, Fe–Ni alloys and pure Ni, have been subjected to the same nitrogen ion beam processing conditions to examine the role of alloy composition in the surface modification behavior. A low-energy (700 eV), high-flux (2 mA cm −2 ) beam of ions was used with each sample held at 400 °C during a 15 min treatment. The near surface regions have been characterized by conventional and grazing-incidence X-ray diffraction, Auger electron spectroscopy, conversion electron Mossbauer spectroscopy, and microhardness measurements. There is a clear distinction in the modifications depending on whether the alloys are Fe-rich or Ni-rich. Fe-rich samples all yield relatively thick (2.5–3.5 μm) layers with high N content in solid solution. The large lattice expansions lead to ferromagnetism in these surfaces. A novel double-layer structure has been induced in all the Fe-rich alloys, corresponding to two rather well-defined N contents: high (20–26 at%) in the surface layer, and medium (4–10 at%) in the subsurface layer. It is suggested that this substructure is caused by stress-assisted diffusion. The Ni-rich alloys have much thinner N-containing layers (≤1 μm) and a much lower amount of N retained in the (111) planes oriented parallel to the surface compared to those in the Fe-rich alloys.

Asymmetric bipolar pulsed DC: the enabling technology for reactive PVD

Abstract: The use of reactive DC sputtering for the deposition of insulators from conductive targets has been limited by the intrinsic problem of target poisoning and the consequent arcing and process instabilities. The need to deposit high quality dielectric films rapidly is becoming more important as technology pushes forward. Asymmetric bipolar pulsed DC eliminates target poisoning through preferential sputtering, enabling existing PVD tools to produce the high-quality, low-defect dielectric films needed for next generation processes. Typical films being produced with asymmetric bipolar pulsed DC from metallic targets include Al2O3, AlN, SiO2, SiN, Ta2O5, DLC, TaN, TiN and ITO. The mechanisms of target poisoning and dielectric arcing are explained in this paper, and solutions are given.

Laser cladding of nickel-based hardfacing alloys

Abstract: The effects of powder feed rate (PFR) and translation speed (TS) of a laser beam were investigated on laser cladding with three nickel-based hardfacing alloys: Colmonoy 6, Colmonoy 88 and AI-1236. The width-to-height ratio of single clad passes and the extent of dilution were found to decrease with increasing PFR and decreasing TS. Microstructurally, Colmonoy 6 clad layers consisted of a nickel-based structure of primary γ-nickel dendrites and interdendritic eutectics of γ-nickel, nickel borides and nickel silicides, Colmonoy 88 of mixed carbides and AI-1236 of partially melted WC particles and mixed carbides embedded in similar nickel-based microstructures. Block-on-ring wear tests showed that the wear mechanism is largely abrasive in nature for all three alloys studied. The wear resistance was found to correlate better with the type and amount of the main hard phase present in the clad layer: AI-1236 clad layers were much superior to those of Colmonoy 6 and Colmonoy 88 clad layers owing to WC particles in the alloy.

Synthesis of pseudobinary Cr-Al-N films with B1 structure by rf-assisted magnetron sputtering method

Abstract: Pseudobinary Cr-Al-N films were synthesized by a new inductively combined rf-plasma assisted magnetron sputtering method. It was found that phase transition from B1 (NaCl) structure to B4 (wurtzite) structure occurs at an AlN content between 70 mol% and 80mol%, and the critical composition for the phase transition showed excellent agreement with the composition (77mol% AlN) predicted by two band parameters. It was found that the aging effect is observed in the pseudobinary nitride films synthesized by the new magnetron sputtering method. No preferred orientation was observed in as-deposited pseudobinary nitride films with B1 structure, while preferred orientation appeared in these films after aging at room temperature for six months.

Influence of heat treatment on the abrasive wear behaviour of HVOF sprayed WC–Co coatings

Abstract: WC–17 wt%Co powders have been sprayed by the HVOF method to form coatings approximately 200 μm thick on steel substrates. The microstructure of the coatings has been analysed and found to consist of WC, W2C and an amorphous binder phase. Some of the carbide particles are found to decarburise and dissolve in the liquid metal binder during spraying, resulting in a brittle binder phase in the coating. The coatings also exhibit tensile stresses in the as-sprayed condition. Coatings have been heat-treated at a range of temperatures between 250 and 1100 °C. Heat treatment above 600 °C results in significant phase changes within the coating. However, heat treatment at all the temperatures examined resulted in changes in the integrity and residual stress state of the coating due to thermal expansion coefficient mismatch between the coating and substrate. The abrasive wear behaviour of the as-sprayed and heat-treated coatings demonstrates that heat treatment improves the wear behaviour. Indeed, heat treatment at temperatures as low as 250 °C has resulted in improvements in wear resistance of up to 35%. The abrasive wear behaviour has been correlated with the residual stress state, coating integrity and microstructure.

Low-pressure, high-density plasma nitriding: mechanisms, technology and results

Abstract: This paper reviews the low-pressure (<10 Pa), high-density plasma-assisted nitriding processes recently developed for metallurgical surface modification to improve wear, hardness and fatigue resistance of ferrous and non-ferrous materials. For that purpose, plasma generation is most frequently ensured by d.c. glow discharges at relatively high pressure (100–1000 Pa) with the underlying limitations associated with this technology. Nevertheless, more flexibility and control are required for plasma nitriding of promising non-ferrous materials such as titanium, aluminium and their alloys. These requirements are fulfilled by the recently developed enhanced or intensified plasma nitriding processes that operate at lower pressures (<10 Pa) such as: thermionically assisted d.c. triode arrangements (TAT), plasma immersion ion implantation (PIII) or plasma source ion implantation (PSII), electron cyclotron resonance (ECR) systems and thermionic arc discharges (TAD). The purpose of this paper is to review these new nitriding processes from both technological and fundamental points of view. Plasma parameters and plasma–surface interactions are considered for these processes.

Parameter studies on high-velocity oxy-fuel spraying of MCrAlY coatings

Abstract: Todays thermally sprayed MCrAlY coatings are commonly manufactured by the vacuum plasma spraying (VPS) process. This technique provides dense and oxide-free coatings. However, mainly due to the vacuum procedures this production is cost intensive and time consuming. The third generation of high-velocity oxy-fuel (HVOF) systems nowadays offer processing of materials that are sensitive to oxidation even in atmosphere. This is mainly due to the achievement of higher kinetic energy of the particulates and lower melting degrees which enables particle flattening in a plastic state. The work presented here focuses the influences of process parameters of a gas-driven HVOF system on the microstructure and oxygen content of MCrAlY coatings. The major parameters were subjected to DOE investigation to estimate both single and interacting effects. It was found that spray distance, fuel/oxygen ratio and powder feed rate exert a major influence on microstructure and oxygen content, whereas powder feed gas rate is not significant. Further parameters of significance are substrate temperature, shroud-gas type, fuel-gas type and powder size fraction. The coatings with the lowest oxygen contents were subjected to heat-treatment in vacuum and oxidation in air. Subsequently they were metallographically investigated. A comparison with VPS coatings showed that HVOF-sprayed MCrAlY coatings show similar oxidation behavior.

(Cr:Al)N coatings deposited by the cathodic vacuum are evaporation

Abstract: (Cr:Al)N coatings were deposited using two cathodic vacuum arc evaporators fitted with chromium and aluminium cathodes. Both monolayer and multilayer coatings were deposited with different aluminium content. Selected mechanical and tribological properties of the coating were investigated as well as the oxidation behaviour. It is shown that the oxidation rate decreases with increasing aluminium content. The hardness of the (Cr:Al)N coating is higher than that of CrN coatings.

Mechanical and thermophysical properties of thick PYSZ thermal barrier coatings : correlation with microstructure and spraying parameters

Abstract: The thermophysical and mechanical properties of plasma-sprayed thermal barrier coatings are very strongly dependent on the microstructure, and this may be controlled by manipulating the parameters controlling the plasma spray process. A series of coatings was produced by plasma spraying ZrO 2 +8wt%Y 2 O 3 (PYSZ) to a thickness of about 2 mm, some six to eight times thicker then state of the art coatings currently in service. A controlled variation of plasma spray parameters was carried out to produce a range of different microstructures that could be related to material properties and these in turn correlated with the process variables. The particular properties studied were the thermophysical properties, diffusivity and expansion, and the mechanical properties, modulus, strain to failure and flexural strength measured in four point bend.

Raman studies of tetrahedral amorphous carbon films deposited by filtered cathodic vacuum arc

Abstract: Raman spectra of tetrahedral amorphous carbon (ta-C) films have been obtained as a function of impinging carbon ion energy. In order to analyze the spectra quantitatively, the Raman spectra were fitted using a least-squares computer program. The relative Raman intensity is found to decrease with increasing sp3/sp2 bonding ratio in the films. In particular, the parameters from the fits show a strong correlation between the relative intensity ratio and the sp3 fraction.

Deposition and characterization of TiAlN and multi-layered TiN/TiAlN coatings using unbalanced magnetron sputtering

Abstract: TiAlN coatings have been known to be superior to other coatings such as TiN and TiCN in protecting tools which may be damaged by high thermal load (high cutting speed). Unfortunately, these coatings normally suffer greater damage than TiN and TiCN in more mechanically influenced processes such as interrupted cutting or slow speed cutting. The present study aims at developing multi-layered TiN/TiAlN coatings which may offer a good compromise between the properties of TiN and TiAlN. Three approaches including shutter control, power supply control, and rotational stage control were used to deposit multi-layered TiN/TiAlN coatings using unbalanced magnetrons. These coatings were then characterized using SEM, GDOS, nano-indention system, and tribometer. It was found that, in general, these multi-layered TiN/TiAlN coatings had lower wear rate than single-layered TiAlN within the tested sliding speeds. At certain sliding speeds, these coatings also had lower wear rate than TiN. In some tests, a thin layer (0.1 μm) of TiCN was coated on multi-layered coatings in an attempt to reduce frictional damage particularly during run-in stage. The result shows that the wear resistance of the TiCN–(TiN/TiAlN) coating was significantly improved especially at low sliding speed.

Improvement in fretting wear and fatigue resistance of Ti–6Al–4V by application of several surface treatments and coatings

Abstract: Application of surface modification methods is expected to be an ideal solution to mitigate fretting damage. In this study, our aim was to improve the fretting wear and fretting fatigue resistance of titanium alloys by using several types of surface treatments and thin films, including shot-peening, ion-beam-enhanced deposition (IBED) CrN films, shot-peening+IBED CrN films as well as IBED CuNiIn films. Results showed that with the application of all the above surface coatings and treatments, the fretting wear and fretting fatigue resistance of Ti–6Al–4V were improved. However, the mechanisms and effects of several surface modification methods to mitigate the fretting damage were quite different. IBED CrN film exhibited the best fretting fatigue performance while the duplex treatment by shot-peening/IBED CrN film exhibited the highest fretting wear resistance. There are four mechanisms which can be used to explain the different fretting performance of these surface treatments and coatings: (1) to induce a compressive residual stress; (2) to decrease the coefficient of friction; (3) to increase the hardness; (4) to increase the surface roughness.

Low-friction TiN–MoS2 coatings produced by dc magnetron co-deposition

Abstract: Coatings of the overall composition TiNx(MoSy)z have been produced by dc co-sputter deposition from a single unbalanced magnetron target composed of TiN and MoS2 halves. Compositional variation was obtained by placing stainless steel substrates at various positions with respect to the target. X-ray diffraction and photoelectron spectroscopy showed that the coatings were composed of distinct TiNx and MoSy phases in the form of a nanodispersive system. Pin-on-disk tribometry and nanoindentation demonstrated that coatings with a hardness exceeding 20 GPa and friction coefficients of about 0.1 could be produced by selecting suitable composition and deposition parameters. These coatings show potential for improved endurance with respect to current MoS2-based low-friction coatings and may be suitable for use in severe conditions, for example, as cutting tool coatings for dry machining.

Influence of laser-produced microstructures on the tribological behaviour of ceramics

Abstract: Ceramic surfaces were microstructured by excimer laser radiation in order to improve the tribological properties under hydrodynamic and elastohydrodynamic sliding conditions. The laser-induced lubricant pockets generated high local pressures and thus caused a thicker lubricant film, allowing an optimal separation of the contact surfaces even at lower sliding velocities. Wear-intensive boundary lubrication at frequent starting and running-out procedures may be minimized. Initial studies investigating the influence of different geometries of lubricant pockets on the film thickness were carried out using a block-on-ring tribometer. The results for alumina and silicon carbide showed that the structuring of the surface topography resulted in a significant improvement in the lubricant film thickness compared with untreated surfaces.

The Characteristics of Alumina Scales Formed on HVOF-Sprayed MCrAlY Coatings

Abstract: Turbine blades are protected against high-temperature oxidation by thermal barrier coating (TBC) systems, which consist of a ceramic top coating (ZrO 2 /Y 2 O 3 ) and a metal bond coating (MCrAlY, M=Ni, Co). At high temperatures and under oxidative conditions between the bond coating and the ceramic top coating an oxide scale is formed, which protects the metal against further oxidation. The oxidation behavior of thermally sprayed MCrAlY coatings is influenced by the coating process and the composition of the metal alloy. This work is concerned with the isothermal oxidation behavior of high-velocity oxygen fuel (HVOF) MCrAlY (M=Ni, Co) coatings. During thermal spraying two elements, yttrium and aluminum, oxidize. As a consequence the HVOF MCrAlY coatings exhibit a microstructure with fine dispersed Al 2 O 3 and aluminum yttrium oxides. Free-standing bodies of HVOF coatings were isothermally oxidized in synthetic air between 850 and 1050°C for different periods. Oxidation experiments show that the oxidation rate of HVOF coatings is two times slower than the oxidation rate of corresponding VPS MCrAlY coatings. The oxidation mechanism is mainly changed in the transient stage (no metastable modification of Al 2 O 3 was formed). It is supposed that the oxide dispersion favors the formation of α-Al 2 O 3 scales. The presence of the fine oxide dispersion also influences the adherence of the oxide scale. The microstructure of the transient oxide scales were examined by X-ray diffraction, scanning electron microscopy and transmission electron microscopy.