Showing papers in "Surface & Coatings Technology in 1989"

A review of microstructure and properties of plasma sprayed ceramic coatings

Abstract: The present understanding of the plasma-sprayed coating formation process is reviewed. The development of porosity, and the very fine voids formed by incomplete inter-splat contact is discussed. The strong relation between porosity and mechanical and thermal properties of ceramic coatings is reviewed. Some of the controlling factors for the grain size and crystal structure within the deposited splats are discussed, particularly in reference to zirconia coatings.

X-ray diffraction studies of physically vapour-deposited coatings

Abstract: X-ray diffraction was used to analyse coatings that had been deposited onto various substrates by a physical vapour deposition process known as sputter ion plating. To gain an overall picture for the evolution of coating microstructure with deposition time and substrate material, several aspects of X-ray diffraction were utilized: lattice parameter determination, line broadening studies, texture (preferred orientation) analysis and residual stress measurements. The lattice distortions observed were resolved into three types: macrostrain, microstrain and pseudo-macrostrain, defined as Types I, II and III respectively. Type I distortions are due to internal stress present in a thin film and this varies with the substrate material and coating density. Based on a simple biaxial stress analysis, it was shown that for titanium nitride coatings this stress consists of two components, one thermal and the other structural in origin. Ananlysis of the residual stress distribution was extended to include the effects of shear stresses and stress gradients. Results are presented which illustrate the errors which may be introduced into the values of the principal stresses σ11 and σ22 by assuming a biaxial stress distribution. The paper then goes on to a description of the origins of Type II and Type III distortions and how these relate to coating microstructure.

High temperature coatings for gas turbine blades: A review

Abstract: High temperature coatings are formed to protect many engineering components from environmental degradation. The operating temperature and the nature of the corrodant dictate the choice of the coating. The coatings can be formed by different methods, but invariably the properties are dependent on the coating process. The life of the coating depends upon the mode of degradation in service. It is also influenced by the nature of the substrate. The need for coatings is well illustrated by their wide use in gas turbines. This review covers the development of the oxidation- and hot-corrosion-resistant coatings for turbine blades. The rationale behind such protection, starting from the nickel aluminide to future thermal barrier oxide coatings, is outlined. It is followed by a discussion of current trends in post-coating modifications to improve the properties of the coatings. The principles underlying the development of these coatings should serve as a useful guide in the choice of coatings for other high temperature applications.

The growth of diamond and graphite from the gas phase

Abstract: Formation et croissance de diamant et graphite a partir de methane a basse pression: lois fondamentales de la nucleation, cinetique de la croissance

Thermal properties of ceria and yttria partially stabilized zirconia thermal barrier coatings

Abstract: Coatings consisting of zirconia alloyed with 12, 15, 20 and 25 wt.% ceria or 4, 6, 8, 10 and 12 wt.% yttria were prepared using the plasma spray process. For each of these coatings measurements were made of thermal expansion parallel to the substrate and of thermal conductivity perpendicular to the substrate. These results are discussed in conjunction with the phase stability of the specimens, as determined by X-ray diffraction. Coatings containing 25 wt.% ceria or greater than 8 wt.% yttria were found to have stable thermal properties. Coatings containing less of these additions exhibited a thermal expansion hysterisis which was most pronounced at the lowest concentrations. The temperature at which the hysterisis occured was found to reduce with increasing additions of alloying agent. This behaviour was also detected for ZrO2−CeO2 as a change in thermal diffusivity during thermal cycling and is attributed to a reversible tetragonal-monoclinic transformation.

The wear mechanisms occurring in abradable seals of gas turbines

Abstract: Metallographic investigations of worn abradable coatings have been carried out to identify and characterize the main wear mechanisms occuring in turbomachine seals. The following mechanisms have been found: cutting, smearing, adhesive transfer, crushing, melting and tribo-oxidation. Three other mechanisms occur without blade-seal interaction: erosion, corrosion and high-temperature oxidation. Using the data from these investigations as a reference, rub tests have been conducted on a specially designed high-speed—high-temperature rig. The many correlations obtained between test values and wear mechanisms allow the generation of ‘wear mechanism maps’ for coatings of the AlSi-plastic family. Mechanism orientated model tests are needed to understand the correlations between structural parameters and coating behaviour during blade-coating interactions. Wear mechanism maps will help modelling coating systems to be able to withstand severe service conditions. Avoiding the onset of adhesive or melting wear and simultaneously favouring cutting wear produces better abradable seals which will help improve both the operational safety and the economy of modern turbomachines.

Two original pieces of apparatus for the preparation of coatings using dynamic ion beam mixing: First applications

Abstract: Two ion-beam-assisted coating systems have been developed: an electron beam evaporator and a sputtering evaporator, these two pieces of apparatus being connected to an ion implanter. Carbides of titanium and silicon were deposited using these techniques. We observed a considerable improvement in the tribological behaviour of the TiC coatings, especially a marked decrease in the coefficient of friction.

Texture and some properties of vapor-deposited diamond films

Abstract: Polycrystalline diamond films have been prepared by plasma-assisted chemical vapor deposition (CVD) from gaseous mixtures of various methane/ hydrogen ratio. The textural and structural changes of the films have been studied by reactive plasma etching, electron microscopy, Raman spectroscopy and density measurement. Changes in elastic constant and thermal conductivity are briefly reviewed. Single crystals of size 100μm with good crystalline quality were prepared, while the polycrystalline films grown under the same condition proved to be defective. Finally, it is suggested that the defects characteristic of CVD diamonds may be caused by double bonds of various forms.

Engineering with surface coatings: The role of coating microstructure

Abstract: There is now great interest in the tailoring of the surface properties of a bulk material by the application of surface coatings, with a view to imparting to that surface specific characteristics which cannot be attained in the bulk material. If the maximum benefits are to be realised for a particular coating-substrate system, it is of fundamental importance that the interrelationship between the coating microstructure and its properties are understood. By reference to work on physical vapour deposited coatings such interrelationships are explored, and the importance of the coating microstructure in dictating many of the physical properties of the coating, for example hardness, adhesion, levels of internal stress, composition etc., are highlighted.

Diffusion chromizing of ferrous alloys

Abstract: The results of an extensive study of the mechanism of formation of chromium diffusion coatings on ferrous alloys are described. The effects of the significant coating variables on the composition, thickness and microstructure of the coatings are discussed. These variables include the pack geometry, substrate composition, type of halide activator, type of “inert filler”, time, temperature and the type of the chromium source material. The diffusion of chromium and chromium plus vanadium are considered. The chromium concentration profiles are seen to deviate from those predicted from the Fe-Cr phase diagram. Mechanisms for the coating formation are described.

Hard coatings by plasma-assisted PVD technologies: Industrial practice

Abstract: The historical background to the main plasma-assisted PVD processes is presented, emphasizing the methods in current commercial use for the production of hard coatings. Industrial practices with regard to cleaning and processing receive special attention.

The effect of N2 partial pressure, deposition rate and substrate bias potential on the hardness and texture of reactively sputtered TiN coatings

Abstract: The N 2 partial pressure, target power and substrate bias potential all have a significant effect on the hardness and crystallographic orientation of TiN coatings prepared by high-rate reactive sputtering on cemented carbide substrates Variations in both the substrate bias potential and the target power had a strong affect on the hardness and texture of the TiN coatings Vickers microhardness values (50gf) of the TiN coatings varied between 970 and 3290 kgf mm -2 as the bias was stepped between 0 and –200 V and the target power between 23 and 100kW The texture coefficient of the TiN coatings with a constant –100 V bias was strongly (111) at the low power level of 23 kW, but the texture became random as the power was increased to 40 kW and above with the same –100 V bias At a fixed power level of 10 kW, the orientation of the TiN coatings changed from (111) at zero bias to (200) between –20 and –100 V, to strongly (220) at –150 and –200 V The hardness of TiN coatings with N/Ti ratios of 070 - 101 produced by varying the N 2 partial pressure during deposition had a narrow range between 3140 and 3400 kgf mm -2 , but the texture coefficient varied from random to strongly (220) the adhesion of the TiN coatings was measured with the scratch test, and a critical load of 80 ± 05 kgf was the same for all samples except for the ones at the lowest target power (23 kW), low biases (0 and –20 V) and the lowest N/Ti ratio (070)

Advances in partial-pressure control applied to reactive sputtering

Abstract: Summary High-rate reactive sputtering (HRRS) requires rapid and careful control of the reactive gas partial pressure to achieve high deposition rates and to maintain stoichiometry in the coating. HRRS of TiN was first achieved with feedback control of the N 2 utilizing a differentially pumped mass spectrometer system to sense the N 2 partial pressure. Recently, a new instrument called the optical gas controller (OGC) that operates at sputtering pressures became available for sensing the partial pressure of the gases in the sputtering atmosphere. The OGC was used during the reactive sputtering of TiO x with partial-pressure control, and the TiO x hysteresis loop exhibited a wide negative slope region which is much larger than the one found during the reactive sputtering of TiN x . Partial-pressure control, compared to mass flow control during oxide reactive sputtering, leads to better control of the process and to a 3.5 times higher deposition rate for highly resistive oxide coatings. With partial-pressure control, the anatase form of TiO 2 has been produced with lattice parameters of a 0 = 3.780 A and c 0 = 9.610 A.

Flow structure of a turbulent thermal plasma jet

Abstract: The flow structure of a commercial plasma spray torch operating with argon in atmospheric air has been examined. Conditional mean velocities and turbulence intensity measurements were made in the turbulent plasma jet produced by a swirl-stabilized d.c. arc using laser doppler anemometry. Velocity measurements conditional on fluid originating from plasma jet and from the entrained air were obtained by alternately seeding only the plasma gas and the surrounding air. These results strongly imply that an engulfment-type entrainment mechanism takes place in turbulent plasma jets as opposed to a simple diffusion process. Turbulence measurements indicate that the velocity fluctuations are non-isotropic, contrary to what has been assumed in most numerical work. The swirl number (the ratio of angular to axial momentum) was also measured for the plasma jet and found to be 0.04. The results of this paper should be useful in furthering our understanding of the fluid mechanics in a plasma jet, as well as the momentum and heat transfer to injected plasma spray particles, and as a needed database for future numerical work.

Investigation of coatings at high temperature for use in turbomachinery

Abstract: Aircraft gas turbines, steam turbines and coal utilization turbines operating under particulate flows are exposed to erosion and performance deterioration. Almost all advanced aircraft turbines today are manufactured with a protective coating on the blades. The particulates in the steam turbines are produced in the boiler from exfoliated oxide scales and carried in the steam to the turbine. In coal utilization turbines, the particulates are by-products of the combustion process. There are many problem areas which need future research to be able to produce better and more durable high-temperature coatings. However, this paper will present only some of the work done at the University of Cincinnati on erosion testing at high temperatures and velocities for different materials and coatings. The testings have been performed with a special high-temperature erosion wind tunnel which simulates the aerodynamic conditions on the blades.

Reactive deposition of tin films using an unbalanced magnetron

Abstract: Summary The paper presents discharge characteristics for a sputtering device operated as a conventional magnetron (CM) or as an unbalanced magnetron (UM). In the UM, with a strong magnetic field of several millitesla reaching the substrates, a double-site-sustained discharge (DSSD) was observed. The UM with the DSSD operation transports a current Is to substrates which is more than 50% of the magnetron current for a wide range of substrate-target distances dS-T up to 150 mm at a pressure pT = 5 Pa and low substrate bias Us = −100 V. Microstructure, composition, crystallographic structure, mechanical properties and colour are reported for TiN films reactively sputtered by the UM at dS-T = 200 mm, pT = 5 Pa, for different Us values between −5 and −100 V and substrate current densities up to 6 mA cm−2. Dense stoichiometric TiN films with a microhardness of 2270 kg mm−2 and critical load Lc = 64 N were obtained at low bias Us = −40 V.

Pulse plating effects in nickel electrodeposition

Abstract: The effects of pulse plating on the mechanical properties and internal stresses of nickel deposits were investigated. The yield and tensile strengths increased as a result of pulse plating especially at high peak current densities because of grain refinement. A reduction in the tensile internal stresses due to pulse plating was greatest at high current densities. This was attributed to relaxation during the off-time and to larger amounts of codeposited hydrogen, which also resulted in a reduction in the tensile stresses with increasing pulse frequency.

High-temperature corrosion of alumina-forming coatings for superalloys

Abstract: The results of recent experiments on the cyclic oxidation and hot corrosion of diffusion aluminide and CoCrAlY overlay coatings on nickel-base superalloys are described. It is shown that platinum additions to aluminide coatings produce a substantial increase in resistance to cyclic oxidation and high temperature (Type 1) hot corrosion but have a rather small effect on resistance to low-temperature (Type 2) hot corrosion. The corrosion of aluminide coatings is also shown to be influenced by the composition of the underlying superalloy. Mechanisms for the above behavior are proposed. The effects of yttrium and hafnium on the cyclic oxidation of CoCrAl are illustrated. The results of scanning and transmission electron microscope characterization of the hot corrosion morphologies of CoCrAlY coatings are discussed and a mechanism to account for the microstructures is briefly described.

Technical note: A magnetron sputter ion-plating system

Abstract: Summary A new type of magnetron sputter ion-plating system has now been fully developed. High rate deposition at low substrate temperatures is now possible. High ion bombardment of the samples during deposition is achieved at low bias voltages giving coatings with good adhesion and structure. Independent tests on coated drills demonstrate the effectiveness of the coatings in reducing wear under accelerated drilling conditions.

Flight service evaluation of thermal barrier coatings by physical vapor deposition at 5200 H

Abstract: Thermal barrier coatings (TBC) have the potential to improve the performance of jet engines and have been used in combustors for over 15 years. However, it is only recently that they have been actively used in the harsh turbine environment on nozzle guide vane platforms. It is now intended to use TBCs on turbine vane airfoils and rotating blades where the maximum payoff will be realized. Much work has been done in the last six years towards this goal. This report will review one such Rolls-Royce program; flight service evaluation of TBCs. It will demonstrate that some TBC systems can survive in the turbine environment for over 16 000 h, but that particulate erosion could be a problem. It will review the service condition of vanes with TBCs applied by physical vapor deposition, and will demonstrate some of the advantages over plasma sprayed coatings such as: (1) longer thermal cycle lives, (2) smoother surface finishes, (3) better surface finish retention, (4) superior erosion resistance.

Plasma CVD diamond deposition in C-H-O systems

Abstract: For fixed carbon concentrations (2 vol.%), the O/C ratio has been systematically varied. Measurements of the resulting plasma optical emission were carried out and discussed in relation to the introduction of additional reaction paths to the formation of atomic hydrogen as the result of new active intermediate species containing oxygen. In turn, the optical emission results were related to extensions in the diamond formation region of deposition parameter space. The formation region was determined indirectly from deposition rate and film characterization by scanning electron microscopy, X-ray diffractometry and Raman spectroscopy of each of the samples.

Electron microscopy of the growth features and crystal structures of filament assisted CVD diamond films

Abstract: Scanning and transmission electron microscopy (SEM and TEM) have been used to examine the topographic and internal features of diamond films grown by the hot filament assisted chemical vapor deposition method. Films were grown under conditions chosen to provide three distinctly different diamond microstructures. One of these film structures is virtually free of stacking faults and twins within the grains. This paper relates internal and external growth features to each other, to Raman spectra and to the growth conditions. The growth features are discussed in terms of growth mechanisms.

Correlation of interface structure with adhesive strength of ion-plated TiN hard coatings

Abstract: Interlayers of titanium have been used to improve the adhesion of ion-plated TiN coatings. However, the mechanisms of the improvement of adhesion remain unclear. In this study, we have applied analytical electron microscopy on cross-section specimens and Auger electron spectroscopy to investigate the correlation of the interface structure with the scratch adhesion of ion-plated TiN coatings with and without titanium interlayers on M50 steel substrates. Our results have shown that the interlayer can reduce the internal stresses at the interface region. Furthermore, this titanium interlayer can transform into TiC phase at high substrate temperature both during and after ion plating. These types of treatment can in turn increase the adhesion of the TiN coatings via a better interface contact and stronger chemical bonds.

Factors affecting the tribological behaviour of thin hard TiN and TiC coatings

Abstract: The use of thin hard coatings, often of ceramic materials, appears to offer the possibility of controlled improvement of the tribological behaviour, surface damage resistance and corrosion protection of many engineering components. However, it is only recently that the full complexities of the behaviour of coated systems are being appreciated. The mechanical response of a coated surface to scratching and asperity penetration is complex, involving some properties intrinsic to the coating ( e.g. coating material and thickness), some controlled by processing ( e.g. coating microstructure and residual stress state), some determined by the substrate ( e.g. flexure relative to the coating and deformation in response to stresses transmitted through the coating) and some controlled by interfacial adhesion. Also, raising the temperature of coated systems, for example either in genuine high temperature applications or by frictional heating during ambient temperature service, can be detrimental to coating performance and integrity in that it may alter the microstructure, create pores which allow environmental ingress, relax the residual stress state of the coating and affect interfacial bonding. Based on the authors' own work with TiN and TiC coated steels, this paper discusses these factors and highlights the problems of tailoring coatings to particular applications, predicting coating performance and assuring quality and reliability.

Reactive deposition of hard coatings

Abstract: The reactive deposition of thin films is analyzed by means of a microphysical model. This model shows the way in which to (i) eliminate the hysteresis effect and (ii) control the deposition process. Special attention is devoted to physical parameters, such as fluxes of ions v i , metallic particles v m and reactive gas particles v r , the energy of particles bombarding the growing film and the substrate temperature which play a fundamental role in the deposition process, and to deposition systems with enhanced ionization. A correlation between (i) process parameters, (ii) microstructure, texture, phase and chemical composition of films, and (iii) resulting film properties is discussed in detail for Ti−N films. A comparison of arc evaporation and magnetron sputtering is also given. The need for equipment providing independent control of the deposition rate and ion bombardment of films is emphasized.

Synthesis of BN films by the plasma CVD with various solids: BH3NH3, H3BO3 and NaBH4

Abstract: We have studied the formation conditions of cubic boron nitride (c-BN) formed by the dissociation of BH 3 NH 3 , H 3 BO 3 and NaBH 4 in the reative gases H 2 or NH 3 , and also by the plasma excitement of the dissociated gases using r.f. induction and tungsten filament heating. It was found in these deposition processes that the tungsten filament heating greatly contributes to the formation of c-BN as compared with the contribution of r.f. induction. The deposition ratio c-BN/amorphous BN (a-BN) in the films deposited with BH 3 NH 3 −H 2 changes with the change of the emission intensity ratio I (Hα)/ I (H 2 *) in the gas plasma, where the wavelengths of the Hα and H 2 * photoemissions are 656 nm and 603 nm respectively. In particular, when the ratio I (Hα)/ I (H 2 *) becomes maximum, the deposition rate of c-BN also becomes maximum. In the H 3 BO 3 −NH 3 and NaBH 4 −BH 3 systems, it was found that the deposition ratio c-BN/a-BN varies with the emission intensity ratio I (N 2 + )/ I (NH 3 *) and I (N 2 *)/ I (NH 3 *), where the wavelengths of the N 2 + , N 2 * and NH 3 * photoemissions are 391 nm, and 406 nm and 564 nm respectively.

Chemically vapour-deposited hard coatings: Applications and selection guidelines

Abstract: Three selected examples of important areas of commercial application for hard chemically vapour-deposited coatings are discussed: (1) coatings for metal working tools; (2) chemically inert coatings; (3) scratch-resistant coatings for watch cases. Selection criteria for potential applications of chemical vapour deposition (CVD) coating are given. In most of these the hardness of the layer must be combined with other properties such as corrosion resistance, ductility, chemical inertness, colour etc. Future developments based on plasma-assisted CVD processes will allow lower substrate temperatures to be used, enabling metastable layers (such as diamond) or fine-grained non-equilibrium deposits to be produced.

Structure and properties of c-BN film deposited by activated reactive evaporation with a gas activation nozzle

Abstract: This paper describes the structure and properties of cubic boron nitride (c-BN) film deposited using activated reactive evaporation with a gas activation nozzle. c-BN films deposited directly on various substrates showed very poor adhesion. It was found that a hexagonal phase of a few hundreda˚ngstroms thick was formed before the c-BN phase appeared and this resulted in poor adhesion. The adhesion characteristics of the c-BN film to the substrate were significantly improved by inserting silicon nitride interlayer. The micro Vickers hardness of a c-BN film of thickness 1.5μm was 5500– 6300 kg mm-2. As verified by a scratch test, a high substrate temperature effected good adhesion. The coefficient of friction for the c-BN film in contact with stainless steel was about 0.35 in air at ambient temperature, whereas at 1.7 × 10-3 Pa at 400 °C it was extremely low, 0.02.

CrN and (Ti,AI)N coatings deposited by the steered arc and random arc techniques

Abstract: Summary This paper introduces CrN and (Ti,AI)N coatings deposited using the random arc and steered arc techniques. When compared with TiN, CrN coatings are characterized by the fact that they can be deposited as thick, very dense coatings of up to 60 μ m. They exhibit high hardness values of up to 2900 HK 0.05, very good toughness, relatively smooth coatings and, even in the range of high coating thickness, lower friction coefficients. At high temperatures they show improved oxidation behaviour in the 400 – 750 °C range, better corrosion resistance in the salt spray test and higher deposition rates. These coatings promise successful applications, in particular in the abrasive wear sector. Like many new coatings, the (Ti,AI)N coatings can be produced using —the arc technique at high deposition rates. A loss of aluminium takes place during coating phase. Maximum hardness values of up to 3400 HK 0.05 are attained for coatings with 12 at.% Al. With increasing aluminium content there is an increase in roughness, which is lower on the whole in steered arc coatings. For uninterrupted cutting (turning), thick (Ti,AI)N random arc coatings on hard metal are even superior to CVD multilayer coatings. In the case of interrupted cutting (milling) with HSS tools, the (Ti,AI)N coatings, in particular the steered arc coatings, are superior to TiN.

The crystallization of diamond

Abstract: Etude de la germination du diamant a haute pression, de la cristallisation heterogene et de la formation homogene a partir d'une phase gazeuse