Showing papers in "Surface & Coatings Technology in 2005"

Enhancing the microstructure and properties of titanium alloys through nitriding and other surface engineering methods

Abstract: Over the last 40 years, the commercial production of titanium and its alloys has increased steadily. Whilst these materials have some very attractive properties, enabling applications in many industries, they are seldom used in mechanical engineering applications because of their poor tribological properties. This paper starts with an introduction to the titanium material and a review of the different types of surface treatment. The processes of nitriding, oxidation and carburizing are among the most popular thermochemical treatments aiming at improving the surface properties of Ti-alloys. Different kinds of nitriding are investigated like plasma nitriding, ion nitriding, and laser and gas nitriding. The kinetics of nitriding and the conditions for the formation of nitrided layers are studied. The influence of the main processing parameters such as temperature, time on the microstructure and the formation of new phases during the processes of nitriding is discussed. Also based on investigations presented in the literature, the effects of nitriding on the microhardness and the corrosion resistance of titanium and titanium alloys are analyzed. The improved mechanical properties, which arise from these thermochemical treatments, are discussed in relation to the potential for applying these alloys to different industries.

Relationship between photocatalytic activity, hydrophilicity and self-cleaning effect of TiO2/SiO2 films

Abstract: Self-cleaning glass can be realized utilizing photo-induced hydrophilicity of titanium dioxide. In order to understand the photo-induced hydrophilic self-cleaning effect, it is necessary to understand the relationship between the mutual effect of photohydrophlicity and photocatalysis. In this paper, the relationship between hydrophilicity, photocatalysis and the self-cleaning effect is investigated. It is found that the TiO2/SiO2 surface can have more hydrophilic activity and less photocatalytic activity, or vice versa by adding different amount of SiO2. It is the synergetic effect of hydrophilicity and photocatalysis that improves and maintains the self-cleaning effect. SiO2 addition increases the acidity which results in the increase of the hydroxyl content in the composite films, with the consequence that the hydrophilicity and photocatalytic activity are increased during UV irradiation thus enhances the self-cleaning effect.

Mechanical properties and machining performance of Ti1−xAlxN-coated cutting tools

Abstract: The mechanical properties and machining performance of Ti 1− x Al x N-coated cutting tools have been investigated. Processing by arc evaporation using cathodes with a range of compositions was performed to obtain coatings with compositions x =0, x =0.25, x =0.33, x =0.50, x =0.66 and x =0.74. As-deposited coatings with x ≤0.66 had metastable cubic structures, whereas x =0.74 yielded two-phase coatings consisting of cubic and hexagonal structures. The as-deposited and isothermally annealed coatings were characterised by nanoindentation, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Cutting tests revealing tool wear mechanisms were also performed. Results show that the Al content, x , promotes a (200) preferred crystallographic orientation and has a large influence on the hardness of as-deposited coatings. The high hardness (∼37 GPa) and texture of the as-deposited Ti 1− x Al x N coatings are retained for annealing temperatures up to 950 °C, which indicates a superior stability of this system compared to TiN and Ti(C,N) coatings. We propose that competing mechanisms are responsible for the effectively constant hardness: softening by residual stress relaxation through lattice defect annihilation is balanced by hardening from formation of a coherent nanocomposite structure of c-TiN and c-AlN domains by spinodal decomposition. This example of secondary-phase transformation (age-) hardening is proposed as a new route for advanced surface engineering, and for the development of future generation hard coatings.

Biomedical applications of diamond-like carbon (DLC) coatings: A review

Abstract: To resist wear, biomedical components require coatings that are exceptionally hard, have low friction, and are bioinert. Diamond-like carbon has been shown to provide this capability and to prevent leaching of metallic ions into the body. There are many ways to deposit such coatings from carbonaceous precursors, and some offer the means to incorporate other elements such as nitrogen, titanium, or silver. All reported tests of the biocompatibility of DLC coatings have been successful. This review will summarize work done on orthopedic and cardiovascular components together with other medical applications. For optimum tribological performance, the DLC must be deposited onto highly polished surfaces. The stage has been set for more simulation tests, leading to clinical trials, but the prospects appear to be very good.

Analysis of coaxial laser cladding processing conditions

Abstract: The formation of thick Ni-based coating on a steel substrate by coaxial laser cladding using the Nd:YAG 2 kW continuous laser was studied both from a theoretical and experimental point of view. The theoretical analysis concentrated on the transfer of laser irradiation and powder particles using a simple model of heat transfer to the substrate. This approach provides predictions of the laser power required for melting powder particles and substrate, respectively. For an appropriate experimental analysis of the main process parameters involved, a method based on a gradual change of a single processing parameter was examined. Correlations between the main processing parameters and geometrical characteristics of an individual laser track have been found and are discussed.

Investigation of the properties of Al1−xCrxN coatings prepared by cathodic arc evaporation

Abstract: Al1−xCrxN coatings prepared by different deposition techniques were investigated in the past, for which excellent oxidation and wear resistance (in comparison to Ti1−xAlxN-coatings) was found. In this work Al1−xCrxN films are deposited from Al1−xCrx targets with various composition (0

Toughness measurement of thin films: a critical review

Abstract: At present, there is neither standard test procedure nor standard methodology for assessment of toughness of thin films. However, researchers have long been trying to make such measurements, thus a spectrum of test methods have been developed, mostly each in its own way. As qualitative or semiquantitative assessment, a simple plasticity measurement or scratch adhesion test can mostly suffice. For quantitative description, however, a choice of bending, buckling, indentation, scratching, or tensile test has to be made. These testing methods are either stress-based or energy-based. This paper gives a critical review on these methods and concludes that, for thin films, the energy-based approach, especially the one independent of substrate, is more advantageous.

Thermo-physical properties of plasma electrolytic oxide coatings on aluminium

Abstract: Plasma electrolytic oxide coatings appear to offer attractive combinations of hardness, wear resistance, corrosion resistance and interfacial adhesion. In order to optimise such characteristics, however, more basic thermo-physical property data are required, together with an understanding of how they are affected by processing conditions and microstructure. In the present study, coatings were produced on 6082 aluminium and characterised using profilometry, scanning electron microscopy, X-ray diffraction and nanoindentation. The in-plane thermal expansivity of detached coatings was measured by dilatometry to be about 8 microstrain K −1 . There is thus a rather substantial mismatch between the expansivities of coating and substrate, amounting to about 15 microstrain K −1 . The global in-plane Young's modulus was estimated using cantilever bending of sandwich coated substrates and also by measuring the curvature generated in a bi-material beam on cooling to low temperature. It was found to lie in the approximate range of 10–40 GPa. Values of this order, which are low compared with the figure of around 370 GPa expected for fully dense polycrystalline alumina, are thought to be associated with the presence of a network of microcracks and voids. A low value is expected to be beneficial in terms of conferring good strain tolerance, and hence resistance to spallation driven by differential thermal expansion.

Toughening of hard nanostructural thin films: a critical review

Abstract: For engineering applications of thin films, appropriate combination of high hardness with other properties (such as high toughness, low residual stress, good adhesion with substrate and oxidation resistance) is of vital importance. Super high hardness alone does not have too much use. For practical application, hardness and toughness are of the same importance. This paper gives a critical review on toughening methodologies for hard nanostructural thin films, these are: ductile phase toughening, nanograin boundary strengthening and sliding, composition and structure grading, multilayer design, carbon nanotube toughening, phase transformation toughening, compressive stress toughening, etc. A summary is given to cap the essence of toughening methodologies in terms of increasing the storage or dissipation of plastic energy.

Corrosion behavior of carbon nanotubes–Ni composite coating

Abstract: Ni–carbon nanotube (CNTs) composite coatings were deposited on a carbon steel by electrodeposition The effect of incorporation of CNTs into the nickel coating on the morphology of the coating surface and corrosion properties was investigated Scanning electron microscopy (SEM) showed that the CNTs appear well dispersed in the nickel layer Corrosion tests were performed in aqueous NaCl (35 wt%) using a weight loss method and electrochemical measurements for bare, pure nickel coated and CNTs–nickel coated samples The results showed that addition of CNTs in the deposition process of nickel significantly increased the resistance to corrosion The anti-corrosion mechanism of the composite coating is also discussed

Oxide ceramic coatings on aluminium alloys produced by a pulsed bipolar plasma electrolytic oxidation process

Abstract: In the paper, a modified Plasma Electrolytic Oxidation process (PEO) to produce ceramic coatings on Al alloys, suitable for tribological applications, is discussed. The process utilises bipolar current pulses in the kHz frequency range, providing better control over plasma discharges occurring at the sample surface. The coatings, formed on a 2024 series Al alloy, are characterized by means of optical microscopy, SEM, EDX, XRD and surface profilometry. Microhardness and scratch adhesion tests are performed to evaluate the coating mechanical performance. The effects of current pulse frequency on both the layer growth kinetics and the process energy efficiency are discussed. It is found that in the 1 to 3 kHz frequency range, the layer growth rate can be increased from 0.5 to 1 to 1.6 to 3.2 μm min−1 and the volume fraction of the porous outer layer can be reduced from 25 to 20% to 15 to 10% of the total layer thickness, compared to the conventional 50 Hz AC PEO process. The inner layer, despite a slight increase in porosity, preserves a relatively high hardness of 1200 to 1500 HK25 and good adhesion (LC2=60 N), which should be sufficient for many tribological applications.

Electroless Ni-B coatings: preparation and evaluation of hardness and wear resistance

Abstract: The present work aims to study the hardness and wear resistance of electroless Ni–B coatings. An alkaline bath having nickel chloride as the source of nickel and borohydride as the reducing agent was used to prepare the electroless Ni–B coatings. The structure, microhardness and wear resistance of electroless Ni–B coatings, both in as-plated and heat-treated conditions, were evaluated using X-ray diffraction (XRD), Leitz microhardness tester and a pin-on-disc wear test apparatus. XRD patterns reveal that electroless Ni–B coatings are amorphous in as-plated condition and undergo phase transformation to crystalline nickel and nickel borides upon heat-treatment. The microhardness of the electroless Ni–B coatings increases with increase in heat-treatment temperature and exhibit two maxima in the hardness vs. heat-treatment temperature curve. The specific wear rate increases with increase in applied load from 20 to 40 N and at all applied loads, the specific wear rate and coefficient of friction are less for heat-treated electroless Ni–B deposits compared to that obtained for as-plated ones. The wear process of electroless Ni–B coatings is governed by an adhesive wear mechanism.

Characterization of sputtered NiO thin films

Abstract: Nickel oxide (NiO) thin films were deposited by RF magnetron sputtering process at different RF powers and substrate temperatures in a pure oxygen atmosphere. The structural, optical and electrical properties of NiO films were investigated using X-ray diffraction (XRD), visible spectrum and Hall effect measurements. The dependences of film properties on substrate temperature, crystalline structure and natural aging effect were studied. The results show that the resistivity increases as sputtering power increases from 100 to 200 W at constant temperature. The lowest resistivity and Hall coefficient obtained are 16.7 V cm and 1.99 cm 3 /C, respectively, as the sputtering power is 100 Wand substrate temperature is 3508. The highest carrier concentration obtained is 3.1310 18 cm 3 as the sputtering power is 100 Wand substrate temperature is 3508. The crystal structure was analyzed by X-ray diffraction method. The preferred orientation of NiO film changes from (111) to (200) when the substrate temperature varies from unheated condition to 3508. Electrical properties of NiO films were unstable and show a natural aging effect. Resistivity of NiO films increases as the time of natural aging increases. Under the substrate-unheated condition, the transmittance of as-deposited samples is lower compared to the film prepared at substrate temperature of 350 8C. The change in transmittance may be due to the microstructural change in the material. It is suggested that the sputtering power affects the preferred orientation of NiO film. Higher substrate temperature induces larger grain size and more perfect crystalline structure, which lead to low resistivity of NiO film. D 2004 Published by Elsevier B.V.

Comparison of nano-indentation hardness to microhardness

Abstract: With a nano-indenter and a microhardness testing machine, nano-indentation hardness and microhardness are measured in a wide load range (0.1–19600 mN) for five materials. Even fused silica and silicon almost have constant hardness during the load range, the nano-indentation hardness of copper, stainless steel and nickel titanium alloy shows obvious indentation size effect, namely that the hardness decreases with the increase of depth. For the measured materials, the nano-indentation hardness is about 10–30% in magnitude larger than the microhardness. The main reasons can be explained as the analysis of the nano-indentation hardness using the projected contact area at peak load A c instead of the residual projected area A r , as well as the purely elastic contact assumption describing the elastic/plastic indentation process. The analysis based on a simple model indicates that A c is always smaller than A r , and the more heavily the indent piles up (or sinks in), the larger the difference between the nano-indentation hardness and microhardness.

Atmospheric pressure plasma deposition of thin films by Townsend dielectric barrier discharge

Abstract: The aim of this study is to contribute to the understanding of the thin film growth mechanism using an Atmospheric Pressure Townsend-like Discharge (APTD). Films obtained in dielectric barrier discharges fed with N2 and small admixtures of hexamethyldisiloxane (HMDSO) and nitrous oxide (N2O) as oxidizer gas have been investigated. Results are compared to those obtained with SiH4, in similar conditions. The discharge dissipated power and the feed composition ([N2O]/[HMDSO] ratio) on film properties have been investigated by means of Fourier Transform Infrared Spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS) and Ellipsometry. The film thickness homogeneity and deposition rate have been measured by means of a profilometer and an ellipsometer at the same time. Then silicon oxide thin film properties obtained with SiH4 and HMDSO containing APTD are compared. Concerning chemical composition results are similar. In the two cases, a rather low adding of N2O allows to get SiOx layer without N and C incorporation. Si–OH bounds are always observed. The relative contribution of homogeneous and heterogeneous growth mechanisms is very dependent on the nature of the precursor. Because of its high reactivity, SiH4 induces particles formation in the plasma. These particles are efficiently included in the coating, decreasing drastically the layer density. Thin films made with HMDSO are always dense.

Fabrication of WC–Co coatings by cold spray deposition

Abstract: Processing of cermet such as WC–Co is not easy by cold (gas dynamic) spraying although cold spray process can eliminate the degradation of the phase as compared to conventional high velocity oxy-fuel (HVOF) spraying process. In this study, WC–12∼17%Co powders with nano- and microstructures were deposited by cold spray process using nitrogen and helium gases. Microstructural characterization and phase analysis of feedstock powders and as-deposited coatings were carried out by SEM and X-ray diffraction (XRD). The results show that there is no detrimental phase transformation and/or decarburization of WC by cold spray deposition as expected. It is also observed that nano-sized WC in the feedstock powder is maintained in the cold sprayed coatings. It seems that nano-sized WC is advantageous over micro-sized WC for cold spray deposition because higher particle velocity can be obtained with the same gas velocity. It is demonstrated that it is possible to fabricate the nano-structured WC–Co coating with low porosity and very high hardness (∼2050 HV) by cold spray deposition with reasonable powder preheating.

Electroless deposition of Ni–Cu–P, Ni–W–P and Ni–W–Cu–P alloys

Abstract: Interest in electroless plating of nickel-based ternary alloys has increased because of their excellent corrosion, wear, thermal and electrical resistance. They also possess good magnetic properties. In the present investigation, the effect of copper and tungsten in alkaline electroless nickel baths has been studied in depositing Ni–Cu–P and Ni–W–P alloys and also the synergistic effect of ions in depositing Ni–W–Cu–P alloys. Deposits were characterized using XRD, scanning electron microscopy (SEM), energy-dispersive analysis of X-ray (EDX) and atomic force microscopy (AFM) techniques. XRD results revealed that not much variation in structure and grain size has been found in Ni–Cu–P deposit. A decrease in phosphorus content and a marginal increase in grain size have been observed due to the tungsten addition in the Ni–P deposit. Addition of copper in Ni–W–P baths has resulted in a quaternary deposit, Ni–W–Cu–P, with increased crystallinity. SEM studies showed that presence of coarse nodules in ternary Ni–Cu–P and Ni–W–P deposits. Addition of copper in Ni–W–P baths has resulted in a very smooth deposit. Studies by AFM on deposits have proved that the copper has suppressed coarse nodules by inhibiting their growth in quaternary deposit. No considerable change in hardness has been noticed in both as-plated and heat-treated deposits due to the inclusion of copper in Ni–W–P deposit. A marginal improvement in corrosion resistance has been observed in quaternary alloy compared to ternary (Ni–Cu–P or Ni–W–P) alloys.

Characterization of anodic films formed on AZ91D magnesium alloy

Abstract: Anodization of die-casted AZ91D magnesium alloy was performed in 3 M KOH+0.21 M Na 3 PO 4 +0.6 M KF base electrolyte with and without Al(NO 3 ) 3 addition. The anodic film was characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The corrosion resistance of the various anodized alloys was then evaluated in 3.5 wt % NaCl solution using electrochemical impedance spectroscopy (EIS) and immersion testing. The results showed that the anodic film was mainly composed of MgO. The addition of Al(NO 3 ) 3 into the base electrolyte results in the formation of Al 2 O 3 and Al(OH) 3 in the anodic film. The maximum amount of Al 2 O 3 was found in the anodic film when the alloy was anodized in the electrolyte containing 0.15 M Al(NO 3 ) 3 . The results of EIS analysis and morphological examination showed that the MgO anodic film modified with Al 2 O 3 exhibited the superior corrosiom resistance for AZ91D Mg alloy.

A crystal chemical approach to the formulation of self-lubricating nanocomposite coatings

Abstract: In recent years, design or formulation of superhard coatings with self-lubricating properties has become one of the hottest research topics in the field of tribological coatings. In particular, the production of new coating architectures based on nano-composite or -layered morphologies has become very popular, especially for dry machining or high-temperature sliding bearing applications. In such applications, these coatings can lead to the formation of self-lubricating tribofilms that reduce friction and wear. In this paper, a crystal chemical model is presented to account for the formation of such low-shear and hence low-friction tribofilms. In support of this model, recent experimental findings of various research groups are also provided. The major benefit of this model is that it may provide a scientific means to better design and formulate future tribological coatings that provide not only superior wear resistance but also sufficient lubricity during dry machining or sliding applications.

Multilayer sol–gel coatings for corrosion protection of magnesium

Abstract: Magnesium, although valuable, is reactive and requires protection before it can be used in many applications. This study evaluates the corrosion protection of Mg by multilayer coatings. The coatings consist of an anodised layer and one or two sol–gel layers. Potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) were used to evaluate the corrosion properties of these coatings. A porous anodised Mg oxide layer is employed as an intermediate layer, which enhances the adhesion of sol–gel layers. Sol–gel layers provide corrosion protection by physically sealing pores in the anodised layer and acting as a barrier. Defects and porosity in the coatings are the main causes of corrosion, allowing diffusion paths for corrosive species to reach the metal surface, where they can initiate corrosion. The multilayer approach is found to significantly improve the corrosion resistance of Mg by reducing levels of porosity.

Hot corrosion of some superalloys and role of high-velocity oxy-fuel spray coatings : a review

Abstract: Hot corrosion is a serious problem in power generation equipment, gas turbines, internal combustion engines, fluidized bed combustion, industrial waste incinerators and paper and pulp industries. No alloy is immune to hot corrosion attack indefinitely although there are some alloy compositions that require a long initiation time at which the hot corrosion process moves from the initiation stage to the propagation stage. Superalloys have been developed for high-temperature applications. However, these alloys are not always able to meet both the high-temperature strength and high-temperature corrosion resistance simultaneously, so the need to be protected from corrosion. The high-temperature protecting system must meet several criteria, provide adequate environment resistance, be chemically and mechanically compatible with the substrate, be practically applicable, reliable and economically viable. This paper briefly reviews the hot corrosion of some Ni- and Fe-base superalloys to understand the phenomenon. Comprehensive reviews of the corrosion of coatings have appeared regularly since early 1970; the purpose of this paper is not to repeat the published materials but rather to focus on research trends and to point out some research prospects. High-velocity oxy-fuel (HVOF) spraying is a new and rapidly developing technology in combating high-temperature corrosion and is now challenging the vacuum plasma spraying technique (VPS), which is very expensive (capital costs approximately 2 million US$). HVOF coatings have very low porosity, high hardness, high abrasive resistance, good wear resistance with a strong ability to resist high-temperature corrosion resistance. The purpose here is to summarize the performance of such coatings on various substrates. The effect of coatings thickness, residual stresses induced in the substrates, pre and postheat treatment of the HVOF coatings have been reviewed with the aims of summarizing their high-temperature corrosion resistance properties.

Corrosion, erosion and erosion–corrosion performance of plasma electrolytic oxidation (PEO) deposited Al2O3 coatings

Abstract: Electrolytic plasma techniques have been used to surface modify 6082 aluminium alloy to a depth of 100 ?m. Potentiodynamic polarisations and electrochemical impedance spectroscopy (EIS) have been made under static conditions in order to evaluate the corrosion performance of both unsealed and sealed PEO coatings. The erosion and erosion–corrosion performance of the unsealed PEO Al2O3 coating has also been assessed for a range erodent kinetic energies which varied between 0.016 and 7.1 ?J. The erosion and erosion–corrosion tests were carried out using subangular sand particles ranging in size from 135 to 235 ?m. Scanning electron microscopy (SEM) was used to investigate the coating microstructure and the coating/substrate interface. The analyses show that the PEO Al2O3 coatings studied in this investigation had a pore/crack network to the Al alloy substrate, which compromised the corrosion performance and ultimately the erosion–corrosion performance.

Structure, mechanical and tribological properties of sputtered Ti1–xAlxN coatings with 0.5≤x≤0.75

Abstract: Ti1–xAlxN hard coatings are already successfully applied for cutting tool applications with extreme conditions. Special emphasis in this field is laid on high hardness, oxidation resistance and superior tribological properties. The aim of this work is to present a comprehensive study on the influence of the Al content on coating microstructure and related mechanical and tribological properties. A DC magnetron sputtering system was used to deposit coatings in an Ar+N2 discharge at constant N2 partial pressure and bias voltage onto high-speed steel substrates. Ti1–xTAlxT targets with atomic ratios xT=0.5, 0.6, 0.67 and 0.75 were used. By X-ray diffraction and transmission microscopy the fcc single-phase coatings at low Al contents and dual-phase or hcp coatings at higher Al contents are investigated in detail. Hardness measurements showed high values of 33 GPa for x=0.54 in the coating, decreasing with increasing Al to values of 19 GPa at x=0.76. Friction coefficients against stainless steel balls were high at room temperature with values around 1.5, but decreased significantly at higher temperatures to 0.88 at 700 °C. The wear performance was better for dual-phase and hcp coatings with high Al contents compared to fcc coatings. This investigation shows clearly the relations between target and coating composition, where the resulting structure specifies their mechanical and tribological properties.

Anodization of AZ91D magnesium alloy in silicate-containing electrolytes

Abstract: The effects of Al(NO 3 ) 3 and/or Na 2 SiO 3 additions in the anodizing electrolyte on the properties of the anodic films formed on AZ91D magnesium alloy were investigated. The base electrolyte used in this investigation was 3 M KOH+0.21 M Na 3 PO 4 +0.6 M KF solution. The crystal structure, chemical state, surface morphology and cross-section microstructure of the anodic film was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The corrosion behavior of anodized Mg alloy is examined using electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl solution. The results show that the anodic film was mainly composed of MgO. The additions of Na 2 SiO 3 and/or 0.15 M Al(NO 3 ) 3 into to the base electrolyte enhanced the formations of Al 2 O 3 , MgAl 2 O 4 in the silicate-containing anodized film, accompanied with silicate formation. Increased corrosion resistance of the anodic film was found when AZ91D Mg alloy was anodized in 3 M KOH+0.21 M Na 3 PO 4 +0.6 M KF+0.15 M Al(NO 3 ) 3 with 0.1 M Na 2 SiO 3 addition.

Characterization of masking layers for deep wet etching of glass in an improved HF/HCl solution

Abstract: The paper presents a solution for improving the quality of the surface generated during deep wet etching of glass using an HF (49%)/HCl (37%) solution in a volumetric ratio 10:1. Pyrex glass (Corning 7740) and soda lime glass were analyzed. In addition, the characterization of the main masking layers, including photoresist, amorphous silicon, polysilicon and Cr/Au for deep wet etching in the optimal solution, is described.

Surface modification of a Ti–6Al–4V alloy by thermal oxidation

Abstract: In this study, the effect of thermal oxidation on the dry sliding wear resistance of a Ti–6Al–4V alloy has been examined. Oxidation has introduced hard surface layers composed of TiO2 and oxygen diffusion zone beneath it. Hardness survey conducted under a load of 10 g with a Vickers pyramid indenter revealed that surface hardness increased from 450 to 1300 HV0,01 upon oxidation at 600 °C for 60 h, which was accompanied by significant improvement in wear resistance. Thus, the dry sliding wear rate of thermally oxidised Ti–6Al–4V alloy was almost negligible when compared to the as-received condition.

The thermal conductivity of plasma electrolytic oxide coatings on aluminium and magnesium

Abstract: Plasma electrolytic oxide coatings have been produced on both aluminium and magnesium substrates. Their microstructures have been studied and deductions made about formation conditions. The thermal conductivities of the coatings have been measured using a simple steady state method. The values obtained are relatively low (~1 W m � 1 K � 1 ). This is explained in terms of the microstructure, which exhibits an extremely fine grain size and a significant proportion of amorphous phase. The porosity levels are low, so the low conductivity is not due to the presence of pores. It is noted that, even with a thickness limit of the order of 100 Am, coatings with such low conductivity may prove useful as thermal barrier layers, particularly since they exhibit excellent adhesion characteristics. D 2005 Elsevier B.V. All rights reserved.

Thermal fatigue cracking of surface engineered hot work tool steels

Abstract: Thermal fatigue cracking is an important life-limiting failure mechanism in die casting tools. It is observed as a network of fine cracks on the surfaces exposed to thermal cycling. The crack network degrades the surface quality of the tool and, consequently, the surface of the casting. Surface engineered materials are today successfully applied to improve the erosion and corrosion resistance. However, their resistance against thermal fatigue is not fully explored. In this work, a selection of hot work tool steel grades was surface modified and experimentally evaluated in a dedicated thermal fatigue simulation test. The surface modifications included boriding, nitriding, Toyota diffusion (CrC), and physical vapour deposition (PVD) of coatings (CrC, CrN and TiAlN), both as single-layers and deposited after nitriding (duplex treatment). Untreated specimens of each tool steel grade were used as references. The test is based on cyclic induction heating and internal cooling of hollow cylindrical test rods. The surface strain is continuously recorded through a non-contact laser speckle technique. Generally, all surface treatments decreased the resistance against surface cracking as compared to the reference materials. The reason is that the engineering processes influence negatively on the mechanical properties of the tool materials. Of the processes evaluated, duplex treatment was the least destructive. It gave a lower crack density than the reference steel, but the diffusion layer is more susceptible to crack propagation. In addition, the single-layered CrN coating showed almost comparable thermal fatigue cracking resistance as the reference material. Finally, the resistance against thermal crack propagation of surface engineered tool steels is primarily determined by the mechanical properties of the substrate material.

Effect of some parameters on microstructure and hardness of alumina coatings prepared by the air plasma spraying process

Abstract: The spraying distance, substrate temperature, coating thickness and surface roughness of substrate during deposition play an important role on the plasma spray coating process and effect the final properties of the coatings. Al 2 O 3 coatings on AISI 304 L stainless steel substrate were prepared to investigate the effects on the coating of these parameters. The results indicated that the parameters such as the spraying distance, substrate temperature, coating thickness and substrate roughness were fairly effected the hardness, porosity and surface roughness of Al 2 O 3 coatings. The lowest surface roughness and the lowest porosity and the highest hardness values of Al 2 O 3 coating were obtained for the spraying distance of 12 cm and the surface roughness of 3.28 μm and the substrate temperature of 500 °C. It also found that the increases of coating thickness were lowered the hardness and enhanced the porosity and the coating roughness.

Oxidation and crack nucleation/growth in an air-plasma-sprayed thermal barrier coating with NiCrAlY bond coat

Abstract: The oxidation behavior of an air-plasma-sprayed thermal barrier coating (APS-TBC) system was investigated in both air and low-pressure oxygen environments. It was found that mixed oxides, in the form of (Cr,Al)2O3·Ni(Cr,Al)2O4·NiO, formed heterogeneously at a very early stage during oxidation in air, and in the meantime, a layer of predominantly Al2O3 grew rather uniformly along the rest of the ceramic/bond coat interface. The mixed oxides were practically absent in the TBC system when exposed in the low-pressure oxygen environment, where the TBC had a longer life. Through comparison of the microstructures of the APS-TBC exposed in air and low-pressure oxygen environment, it was concluded that the mixed oxides played a detrimental role in causing crack nucleation and growth, reducing the life of the TBC in air. The crack nucleation and growth mechanism in the air-plasma-sprayed TBC is further elucidated with emphasis on the Ni(Cr,Al)2O4 and NiO particles embedded in the chromia.