Showing papers in "Surface & Coatings Technology in 1993"

Pulsed magnetron sputter technology

Abstract: At present, a new development trend is becoming evident in which the introduction of a pulsed mode for magnetron sputtering seems to be very promising with respect to process stability and layer quality. For instance, the deposition of TiN requires a plasma density that cannot be attained in normal operation. Here the pulsed mode allows a higher plasma density to be obtained without exceeding the thermal rating of the substrate. In this way, for example, it has been4possible to attain bias currents up to 10 mA cm−2. By using several pulse-controlled small magnetron sources in an array, the alloy composition or film thickness distribution can be adjusted by electronic means. The ever-growing demand for sandwich structures and gradient layers opens new applications for pulsed sputter technology. The reactive deposition of highly insulating layers, e.g. SiO2, TiO2 and Al2O3, has turned out to be rather unsatisfactory up to now. Randomly grown insulating layers on target, anode and plasma-confining electrodes prevent a stable operation. The disturbing effects caused by drifting potential distributions and arcing can be restricted by introducing the pulsed mode in the medium frequency range. With Al2O3, for example, defect densities due to arcing were reduced by three to four orders of magnitude. At rates of 240 nm min−1, the coating operation could be maintained for many hours. A review is given on the present state of activities in the field of pulsed magnetron sputtering. Based on practical examples, the capabilities inherent in this technique are discussed and further development trends derived. In our opinion, pulsed magnetron sputter technology is an innovation of great practical significance.

Microstructure, corrosion and tribological behaviour of plasma immersion ion-implanted austenitic stainless steel

Abstract: The surface modification of AISI 316 stainless steel by plasma immersion ion implantation (PI3) has been investigated over a range of treatment temperatures. Below 250°C the results are similar to those obtained by conventional ion beam implantation of nitrogen, but the depth of nitrogen penetration increases dramatically with temperature. Up to 450 °C a nitrogen-expanded austenite phase is formed which is shown to have improved corrosion performance over the untreated material. At 520 °C chromium nitride is precipated and the expanded austenite transforms to martensite, leading to a reduction in corrosion resistance. Pin-on-disc testing indicates improved wear resistance at all temperatures, with reduction in the wear volume by factors of several hundred at high loads. This can be attributed to the formation of an oxide layer which prevents the initiation of severe metallic wear.

Reactive unbalanced magnetron sputter deposition of polycrystalline TiN/NbN superlattice coatings

Abstract: An opposed dual-cathode unbalanced magnetron sputtering system was used to deposit polycrystalline TiN/NbN superlattice coatings, with periods between 2.5 nm and 150 nm, on M2 tool steel substrates. Analytical techniques including X-ray diffraction. Auger spectroscopy and transmission electron microscopy were used to characterize the structure of the superlattice coatings. These showed that well-defined TiN and NbN layers were obtained, but the interfaces were not perfectly planar or abrupt. The mechanical properties of the superlattice coatings were characterized using a Vickers' microhardness tester and scratch tester. Microhardness values for the 6 μm thick superlattice coatings ranged from 1800 to 5200 kgf mm−2 (Hr 0.05) and were strongly dependent on several deposition parameters such as superlattice period λ, nitrogen partial pressure, the ratio of gas flow to each target, and the negative substrate bias Vs. The highest hardness values were obtained for λ≈6 nm and Vs=150 V. The scratch adhesion critical load of superlattice coatings ranged from 1.8 kgf at Vs=200 V to 11 kfg at Vs=34 V.

Deposition of wear resistant coatings based on diamond like carbon by unbalanced magnetron sputtering

Abstract: Amorphous hydrogenerated carbon films containing a small amount of metal (Me:C-H) have been deposited by closed field unbalanced magnetron sputter ion plating. The films have graded film compositions to optimise the adhesion to the substrates, and multilayer TiC/Ti:C-H films have also been deposited. The films have excellent properties: very high measured microhardness (more than 4000 H v ), excellent adhesion ( L c 115–125 N), coefficients of friction against WC of less than 0.2, and volumetric wear rates one fifth that of titanium nitride. The coating procedure is ideally suited to the system used, and many applications for the films are already realised.

Low temperature plasma diffusion treatment of stainless steels for improved wear resistance

Abstract: The use of a thermionically enhanced low pressure plasma process to provide nitriding and carburizing treatments on a precipitation hardening martensitic stainless steel (AISI 17/4PH) at low temperature (420°C or less) is reported. The resulting diffusion layers are analysed by glow discharge optical spectroscopy and X-ray diffraction depth profiling to provide information on the changes in diffused-species concentration and metallurgical structure with treatment depth. It is shown that there exists the possibility to synthesize a variety of layered structures under different plasma conditions using the low pressure enhanced plasma process—with particular emphasis on the production of hard, yet “precipitate-free” surface layers based on an expanded austenite lattice. These types of layered diffusion treatments may provide considerable improvements in the wear resistance of stainless steels without significantly compromising their desirable corrosion-resistant properties.

Using nanoindentation techniques for the characterization of coated systems: a critique

Abstract: Ultralow load indentation testing, or nanoindentation, has considerable potential for studying the near-surface mechanical properties of solids and seems especially suited to the mechanical characterization of thin-film-coated systems where both low contact loads and high spatial precision can be advantageous. This paper assesses the current possibilities for, and limitations of, nanoindentation for assessing the properties of coated systems. In particular, the pursuit of hardness values alone is questioned, especially when the continuous load and depth data from the indentation cycle provide a far more complete mechanical “fingerprint” from which a number of important parameters describing the system behaviour can be calculated. The further difficulty of assessing the properties as a function of the depth into the coated system is also demonstrated. The additional information accessible through post facto microstructural characterization of the indentations themselves, the possible use of various indenter geometries and the need for parallel computer modelling to predict response are also discussed.

Plasma-Enhanced Magnetron-Sputtered Deposition of Materials

Abstract: Plasma-enhanced magnetron-sputtered deposition (PMD) of materials is employed for low-temperature deposition of hard, wear-resistant thin films, such as metal nitrides, metal carbides, and metal carbo-nitrides, onto large, three-dimensional, irregularly shaped objects (20) without the requirement for substrate manipulation. The deposition is done by using metal sputter targets (18) as the source of the metal and immersing the metal sputter targets (18) in a plasma (16) that is random in direction and fills the deposition chamber (12) by diffusion. The plasma (16) is generated from at least two gases, the first gas comprising an inert gas, such as argon, and the second gas comprising a nitrogen source, such a nitrogen, and/or a carbon source, such as methane. Simultaneous with the deposition, the substrate (20) is bombarded with ions from the plasma (16) by biasing the substrate (20) negative with respect to the plasma (16) to maintain the substrate temperature and control the film microstructure. The substrate (20), metal targets, and plasma (16) are all electrically decoupled from each other and from walls (14) of the deposition chamber (12), so as to provide independent electrical control of each component. The PMD process is applicable not only to the deposition of hard coatings, but also can be applied to any thin film process such as for electrically and thermally conductive coatings and optical coatings, requiring simultaneous, high-flux, ion-bombardment to control film properties.

Microstructure and hydroabrasive wear behaviour of high velocity oxy-fuel thermally sprayed WCCo(Cr) coatings

Abstract: Sand erosion tests were performed on WC-Co and WC-CoCr coatings deposited by the high velocity oxy-fuel spraying method. Several analytical techniques, including X-ray diffraction, Auger electron spectroscopy and energy-dispersive spectroscopy in a transmission electron microscope were used to characterize the microstructures formed during powder processing and spraying. It was found that a substantial fraction of WC decomposed into W2C or reacted with the cobalt matrix to form ternary carbides such as Co3W3C and other mixed compounds. In both cases the binder phase had a nanocrystalline structure of size 4-8 nm containing tungsten, cobalt, carbon and chromium elements. The addition of chromium inhibits to a large extent the decomposition of WC and avoids the formation of metallic tungsten. In addition, chromium improved the erosion resistance by several times compared with the WC-Co coating. Scanning electron microscopy showed that the CoCr matrix binds carbides better than the cobalt matrix, thereby inhibiting carbide loss at the spray particle boundaries. The hydroabrasive wear behaviour of coatings and the mechanisms for material removal are discussed with respect to the microstructures formed during spraying.

Preparation of W-C:H coatings by reactive magnetron sputtering

Abstract: The reactive d.c.-magnetron sputter technique in conventional and unbalanced modes was used to prepare tungsten-containing hydrodcarbon (W-C:H) coatings which have low friction coefficients and high wear resistances. Reproducible deposition processes stable over a long time without poisoning could be realized using a plasma emission monitor (PEM) control unit. From investgigation of light emission spectra of the magnetron discharge the intensive tungsten line at 401 nm was selected as most suitable for process control. The dependences of characteristic process parameters such as target voltage, optical emission intensity or deposition rate on the reactive gas flow were quite different from those known for the reactive sputter deposition of TiN or other nitrides and oxides. For both tungsten and WC targets the PEM intensity decreased monotonically with increasing acetylene flow. However, the deposition rates on substrates with floating potential increased. An additional r.f. excitation of the substrate electrode caused only slight changes in rate and metal content in the coatings. It can be concluded that the sputter process is dominant for W-C:H growth. The contribution of direct plasma polymetrization at the substrate is small. From-X-ray diffraction investigations it was concluded that the W-C:H coatings contain microcrystallites of the metastable cubic tungsten carbide WC1 − x. The Vickers hardness and Young's modulus depend on the atomic ratio of tungsten to carbon.

Plasma-sprayed coatings for titanium alloy oxidation protection

Abstract: The behavior of several different types of plasma-sprayed coatings applied to typical α + β titanium alloys was studied in air at elevated temperatures. Ductile coatings of MCr and MCrAlY type were found to confer protection against oxidation and embrittlement for hundreds of hours during rapid thermal cycling in flowing air to at least 850 °C. Measurements of the extent of interdiffusion of elements across the coating-alloy interface as functions of time and temperature were made. Typically, cobalt and iron from a coating may penetrate to depths of a few tens of micrometers into the alloy substrate after 1000 h exposure at 850 °C.

Oxidation resistance of PVD Cr, Cr-N and Cr-N-O hard coatings

Abstract: The plasma-beam-sputtering process was used to deposit coatings 3 μm thick of various compositions from pure chromium to stoichiometric CrN and a few CrN x O y coatings on steel and sapphire substrates at a temperature of 200°C. The formation of the coatings, their microstructure, morphology, composition and microhardness were determined. Their oxidation behaviour in an oxygen flow in an oven was studied at temperatures between 500 and 800°C by the weight gain method, X-ray diffraction and Auger electron spectroscopy depth profile analysis. The surface morphology and crystallite growth were observed by scanning electron microscopy and roughness measurements. The results show a pronounced minimum in weight gain of Cr-N x coatings as a function of partial pressure of nitrogen after heating in oxygen at 700 and 800 °C. The protective layer on all the Cr-based coatings studied was a stable Cr 2 O 3 oxide. Of all the hard coatings investigated, the stoichiometric CrN coating had the lowest oxidation rate in pure oxygen, also after a long-term test of 200 h. Oxidation in oxygen also induces the growth of crystallites, 0.05 μm in size in as-deposited coatings. After heating for 4 h at 700 °C in oxygen, the grain size increased to 0.4 μm, while at 800 °C it increased to 0.75 μm. There was no essential difference in grain growth between tool steel and sapphire substrates.

Deposition of graded alloy nitride films by closed field unbalanced magnetron sputtering

Abstract: The development of a deposition system based on closed field unbalanced magnetron sputter ion plating (uniform deposition and plasma), has greatly increased the number of complex nitrides that it is possible to routinely deposit. The new alloy nitrides can increase the hardness of standard TiN by a factor of 2, and can consequently result in longer tool life. In order to maintain high levels of coating to substrate adhesion, it is necessary to gradually grade the composition of the film in the interface region. Grading of an alloy nitride such as TiZrN will ensure minimum Lc scratch adhesion levels of 75 N. The technique can be closely controlled and is very flexible, enabling the changes in composition to be attained with precision.

Effect of composition and thickness on corrosion behavior of TiN and ZrN thin films

Abstract: The corrosion behavior of TiN and ZrN coatings at thickness from 5 to 10 μm on AISI 304 stainless in 0.5 N NaCl solution was investigated. Electrochemical impedance spectroscopy, scanning electron microscopy and X-ray photoelectron spectroscopy were the techniques applied to characterize fully the corrosion behavior. From the data, TiN acts as an inert coating, while the ZrN coating reacts with the environment. It is proposed that the nitrogen in the layer is replaced by oxygen, forming a near-surface film of zirconium oxide. It is this film which possesses good corrosion resistance and is responsible for the corrosion protection thought to be afforded by the ZrN layer initially deposited on the surface.

A comparison of the wear and fatigue properties of plasma-assisted physical vapour deposition TiN, CrN and duplex coatings on Ti-6Al-4V

Abstract: The study sets out to establish a comparison between duplex systems of plasma nitriding followed by plasma-assisted physical vapour deposition (PAPVD) of TiN deposited on Ti-6Al-4V, compared with PAPVD of TiN and CrN alone. The fatigue resistance has also been examined since conventional surface modifications can often impair fatigue resistance. A rubber-wheel-type abrasion tester and pin-on-disc sliding wear tester were used to examine the wear resistance of the coatings and the load-bearing capacity of the substrate respectively. A Wohler-type rotating tester was used to study the fatigue properties. Using smooth rotating-bending fatigue specimens tested in air at 5700 rev min-1 for 106 cycles or until failure it was found that TiN, CrN and the duplex coating did not impair fatigue resistance but actually improved the S-N curves and increased the endurance limit. From the wear results it was found that, although TiN and CrN do improve the wear resistance of Ti-6Al-4V significantly, it is the duplex coating that has much the greater load-bearing capacity on the titanium substrate and gives a significant improvement on PAPVD TiN or CrN coatings in sliding and abrasive wear conditions.

Commercial applications of ion beam deposited diamond-like carbon (DLC) coatings

Abstract: We report the results of applications development by Diamonex on ion beam deposited diamond-like carbon (DLC) coatings. Many of the process scale-up issues for ion beam deposition of DLC coatings have been solved. Commercial applications for abrasion resistant DLC-coated optics take advantage of the transparency, chemical inertness, low friction coefficient, high hardness and excellent adhesion of ion beam deposited DLC coatings. DLC-coated polycarbonate sunglass lenses are more resistant to abrasion by #0000 steel wool than any polycarbonate sunglasses commercially available. Using DLC, highly durable coatings with a variety of iridescent reflected colors can be produced. In addition to providing polycarbonate with substantially improved abrasion and chemical resistance, the ballistic strength of polycarbonate is maintained. DLC-coated float glass windows in supermarket laser bar code scanners demonstrated a projected average lifetime of 9 years, approximately 100 times longer than uncoated glass. ZnS and ZnSe IR windows coated with DLC coatings 1–2 μm thick are highly transparent from 3 to 15 μm wavelength and exhibit large improvements in chemical resistance and abrasion resistance relative to uncoated ZnSe and ZnS. The combination of the high electrical resistivity, relatively high thermal conductivity, low friction coefficient, high hardness and excellent adhesion to metals of ion beam deposited DLC, permits its use as a scratch-resistant dielectric layer for copper heat removal devices in multichip modules.

Mechanical properties and microstructures of polycrystalline ceramic/metal superlattices: TiN/Ni and TiN/Ni0.9Cr0.1

Abstract: Polycrystalline superlattice coatings of TiN/Ni and TiN/Ni 0.9 Cr 0.1 of thickness 2–3 μm were deposited onto tool steel substrates using an opposed-cathode reactive unbalanced magnetron sputtering system. The TiN/Ni superlattices have repeated periods (Λ) from 1.8 to 62 nm with various Ni layer thickness (l Ni ) to Λ ratios, l Ni /Λ. TiN/Ni 0.9 Cr 0.1 superlattices have Λ =1.2–7.4 nm with l NiCr / Λ =0.3. The structures were analyzed by X-ray diffraction, as well as plan-view and cross-sectional transmission electron microscopy. The hardnesses and elastic moduli of the superlattices were measured using nanoindentation techniques. An increase in hardness with decreasing Λ and grain size was observed. A maximum hardness of 3500 kgf mm −2 for TiN/Ni, about 1.5 times of the rule-of-mixtures values, was found at l Ni / Λ =0.16 and Λ =2.2 nm. For TiN/Ni 0.9 Cr 0.1 , a maximum hardness of 3200 kgf mm −2 was found at l Ni / gL =0.3 and Λ =1.2 nm. No significant variations in TiN/Ni 0.9 Cr 0.1 film modulus were found as a function of Λ.

Surface modification of polypropylene in oxygen and nitrogen plasmas

Abstract: We have studied in detail the surface modification of polypropylene (PP) in low pressure plasmas of oxygen or nitrogen. As revealed by electron spectroscopy for chemical analysis, contact angle measurements and scanning electron microscopy, the plasma ensures a quick functionalization of the PP surface with O- and N-containing chemical groups within a few seconds, while a roughening of the surface topography takes place on a distinctly enlarged time scale. The plasma configuration and processing conditions are found to be key parameters for achieving a most effective and economical plasma treatment process. When plasma treated under optimized conditions, PP shows an excellent performance in application tests ( e.g. adhesive polymer-metal bonding, adhesion of water-based coatings).

Metallurgical study of low-temperature plasma carbon diffusion treatments for stainless steels

Abstract: We recently reported a novel low-temperature carbon diffusion technique forsurface hardening of stainless steels. The treatment was shown to provide benefits in terms of abrasive wear resistance. There is also evidence to suggest that by performing diffusion treatments at low temperatures ( i.e. below 400°C), these benefits can be achieved without compromising corrosion resistance. Here a variety of surface analysis and depth profiling techniques have been used to determine the physical and mechanical properties of carbon-rich layers produced on a range of stainless steel substrate materials. X-ray diffraction (XRD) was employed to determine the crystallographic structure, whilst wavelength dispersive X-ray analysis (WDX) and glow discharge optical spectroscopy (GDOS) gave information on the concentration and distribution of the diffused species within the treated layers. A variety of carbide-based structures was detected, including the expected M 23 C 6 and, more surprisingly, M 3 C. Optical and electron microscopy techniques were used to provide information on layer morphology. The surfaces produced by the low-temperature carbon-diffusion process generally exhibit a distinct diffusion layer of between 1 and 20 μm, depending on the material and the treatment conditions. Austenitic stainless steels appear to give the best response to treatment, however other types of stainless steel can be treated, particularly if the microstructure contains above 5% retained austenite. Here we discuss the changes in mechanical and metallurgical properties provided by this technique and its potential value for treatment of both austenitic and other stainless steel substrate materials.

Aspects and results of long-term stable deposition of Al2O3 with high rate pulsed reactive magnetron sputtering

Abstract: Several essential conditions for the long-term stable deposition of highly insulating layers by reactive magnetron sputtering from metal targets are studied The following aspects are considered: design of the magnetron and of the deposition arrangement, electric potential of the electrodes surrounding the plasma, position of the gas inlet, appropriate control of the gas inlet, power supply and process stabilization method The defect level of the deposited layers could be reduced by several orders of magnitude compared with layers deposited with a normal magnetron arrangement as a consequence of arc suppression on a microsecond time scale When the magnetron discharge is pulsed at medium frequency (10–200 kHz), the deposition process stabilizes and the arcing tendency is decreased The deposited Al 2 O 3 layers had a maximum thickness of 50 μm The dependences of the deposition rate up to a maximum of 240 nm min -1 and several other film properties ( eg optical and mechanical) on process parameters such as the power density at the target, the reactive gas flow and the discharge voltage were investigated

Process and advantage of multicomponent and multilayer PVD coatings

Abstract: This paper gives a short review of physical vapour deposition (PVD) processes suitable for the deposition of wear-protective films with complex compositions and structures. It then characterizes the advantages of multicomponent and multilayer coatings, and describes their behaviour in cutting tool applications—an area in which PVD coatings are widely used in industry. The research results presented indicate a great future application potential for PVD coatings with complex compositions and structures. Particularly promising in this respect are films which achieve significantly prologned tool lives in interrupted-cut machining, such as Ti-Zr-N, Ti-C-N and TiN/Ti-C-N multilayer coatings.

Diffusion barrier design against rapid interdiffusion of MCrA1Y and Ni-base material

Abstract: A physical vapour deposition, diffusion barrier coating in the system A1-O-N offers the possibility to reduce the interdiffusion between the layers of MCrA1Y and the Ni-base material. The barrier function depends on the high temperature stabilization of the amorphous structure. While amorphous A1-O undergoes modification at 1100°C from several crystalline substructures up to the ϵ phase, A1-O-N remains in the amorphous structure at 1100°C. This coating acts successfully as a passive diffusion barrier, and was tested up to 400 h at 1100°C in annealing tests and up to 2500 cycles in a thermal fatigue test rig with 1115°C and 200°C as the high and low peak temperatures. The gradient-free transition step achieved in the element analysis of the depth profiles, together with the inspection of the cross-sections, confirm the excellent barrier performance.

Characteristics and osteoconductivity of three different plasma-sprayed hydroxyapatite coatings

Abstract: Plasma-sprayed hydroxyapatite coating (HAC) on a bioinert metal substrate was used clinically to increase the fixation of an orthopaedic implant. This study aimed to clarify in vitro various characteristics of plasma-sprayed HACs, and to evaluate in vivo the effect of coating characteristics on the osteoconductivity of HACs. Three different HACs on Ti-6A1-4V substrate were prepared by varying the plasma spraying parameters; these were then characterized in detail. Subsequently, in the cortex of canine femur, a quantitative histologic evaluation was performed. This was done to determine the osteoconductivity of HACs, represented as the new bone healing index (NBHI), after 2, 4, 6, and 12 weeks of implantation. The results in vitro demonstrated that the microstructure, phase composition, crystallinity, OH-ion content, and calcium to phosphorus molar ratio of the HACs varied with the spraying parameters. The HAC with denser microstructure and less thickness showed a higher bonding strength at the HAC-Ti-6A1-4V interface. However, this denser HAC underwent adverse biological degration in terms of NBHI after 12 weeks of implantation because of other characteristics of the coating. Based on the results of the study, it can be concluded that an HAC with both high bonding strength and good osteoconductivity is difficult to acheive.

Coatings resistant to erosive/corrosive and severe environments

Abstract: Advances in materials performance require the development of composite systems, of which coated materials are one form. Abrasion and corrosion resistance of components can be greatly increased by protective coatings and this is a growing industry of considerable economic importance. Coatings are used in both aqueous and high temperature applications. Coal gasification electric power generation and waste incineration involve severe conditions and thick coatings have proved effective. Diesel and gas turbine engines are subject to high temperature corrosion and highly beneficial coatings have been developed. Some nuclear power systems also rely on coatings. Problems include substrate compatibility, adhesion, porosity, possibility of repair or recoating, interdiffusion, effect of thermal cycling, wear and corrosion resistance, and cost. It is essential to design the substrate + coating as an integrated system with all these features in mind.

Tungsten and tungsten-carbon PVD multilayered structures as erosion-resistant coatings

Abstract: The aerospace industry is faced with a major erosive wear problem of engine components operating in dust environments. Various physical vapor deposition (PVD) or chemical vapor deposition (CVD) protective coatings have been used previously with limited success. The present paper deals with the elaboration and characterization of W/W-C multilayer coatings produced by magnetron sputtering from a W target in Ar and in an Ar+methane mixture respectively. This low temperature PVD process, compared with CVD processes, enables the deposition of protective coatings on Ti6A14V substrates at temperatures below 400°C. The microhardness of W-C films was found to be strongly dependent on the carbon concentration; two maximum hardness values of 26 000 MPa were obtained with W-C layers containing either 14–15 at.% or 40–45 at.%°C. This change in microhardness with the carbon content was correlated with the evolution of the crystallographic structure. Multilayer coatings of total thickness 60 μm composed of various W/W-C stacking arrangements have been elaborated with hard W-C films containing 14–15 at.% or 40–45 at.%C. These samples were subjected to erosive wear tests using a sand blast unit. As a comparison, Mo/Mo-C and Cr/Cr-C structures containing more than 10 at.%C were also tested. W/W-C coatings with an appropriate stacking arrangement and a C concentration of 14–15 at.% in W-C layers exhibited an erosion resistance improved by more than two orders of magnitude compared with that of uncoated Ti6A14V substrates. Complementary tests of the fatigue performance of Ti6A14V specimens coated with these very promising erosion-resistant coatings were also performed.

Atomic oxygen protective coatings for Kapton film: a review

Abstract: Low earth orbit (LEO) space missions have shown that an ambient atomic oxygen (AO) environment interacts with spacecraft materials, resulting in surface erosion and significantly affects the performance of the materials, particularly polymeric films. To ensure durability, the vulnerable polymeric film surfaces should be either replaced with AO-resistant materials or protected by applying suitable coatings. This review discusses briefly the degradation mechanism of Kapton film as a result of AO attack. The requirements for the selection of protective coatings for Kapton flexible solar array blankets are discussed. Different groups of materials presently being used as protective coatings are outlined. This is followed by a discussion of the current status of the coating materials development, application processes, performance and their durability in protecting Kapton film from the synergism of the LEO environment. The emphasis is placed on developing new AO-resistant materials. Ground simulation testing requirements are reviewed briefly. This review serves as a useful guide for the selection of a protective coating for Kapton flexible solar array blanket applications.

Reactive unbalanced magnetron sputtering of the nitrides of Ti, Zr, Hf, Cr, Mo, Ti-Al, Ti-Zr and Ti-Al-V

Abstract: Reactive unbalanced magnetron sputtering was used to deposit eight different nitride coatings on hardened 440C stainless steel rolling contact fatigue (RCF) test specimens. The target materials used in this study were Ti, Zr, Hf, Cr, Mo, Ti0.5Al0.5, Ti0.5Zr0.5 and Ti-Al-V (the aircraft alloy Ti-6 wt.%Al-4wt.%V). All the coatings were characterized using X-ray diffraction, microhardness and scratch adhesion tests. Ti, Zr and Hf form the simple binary nitrides of TiN, ZrN and HfN, respectively, with an f.c.c. structure and a range in stoichiometry. Cr and Mo form a range of nitrides, starting with a solid solution of nitrogen in each of them, a Cr2N or Mo2N phase, and a CrN or MoN phase. The MoN phase was not conclusively formed in this work. Mo2N forms in two different phases, i.e. β and γ phases, depending on the nitrogen partial pressure. The Ti-alloyed nitrides all form an f.c.c. structure. The coatings selected for the RCF tests ranged in hardness from a low of 1900 kg mm−2 for Ti0.5Al0.5N and CrN to a high of 2800 kg mm−2 for Ti0.5Zr0.5N. The deposition conditions for forming these nitrides are given in this paper. Also, the similarities and differences, both in the deposition conditions and in the resulting properties, are reviewed.

Electron cyclotron resonance plasma deposition of cubic boron nitride using N-trimethylborazine

Abstract: N -Trimethylborazine has been used as precursor in a downstream electron cyclotron resonance (ECR) plasma process to deposit cubic boron nitride (c-BN). N -Trimethylborazine ((CH 3 -N-B-H) 3 ) is a non-corrosive and non-explosive liquid with a low toxicity. As plasma gas an argon-nitrogen mixture was used and N -trimethylborazine vapour was fed into the downstream region of the ECR plasma source. BN deposits on silicon (111) were characterized by IR spectroscopy, electron probe microanalysis and X-ray diffraction. The formation of nanocrystalline c-BN depends strongly on the process parameters and requires a substrate temperature of above 800°C. Furthermore, the application of a negative substrate bias—in our experiments achieved with a low frequency (100–450 kHz) generator—is essential to increase the c-BN fraction of the deposit. As shown by IR spectroscopy, a stepwise transition from hexagonal BN into wurtzite-type BN and finally into c-BN takes place by changing the deposition conditions. From these observations some conclusions concerning the growth mechanism of c-BN can be derived. Owing to the merits of N -trimethylborazine, its processing—compared with that of diborane or boron trihalides—is uncomplicated and promising for future applications of c-BN.

Superlow friction of oxygen-free MoS2 coatings in ultrahigh vacuum

Abstract: Low friction of MoS 2 -based coatings in the absence of reactive gases and particularly water vapour is generally attributed to friction-induced orientation of “easy shear” planes of the lamellar structure, parallel to the sliding direction. It has been suggested that the substitution of sulphur by oxygen in the MoS 2 structure could improve its tribological performance by increasing the basal plane distance. To check the role of the presence of oxygen in the friction of MoS 2 , we have developed an Auger electron spectroscopy-X-ray photoelectron spectroscopy ultrahigh vacuum tribometer, coupled with a preparation chamber, which allows the investigation of oxygen-free MoS 2 sputter-deposited coatings and the performing of in-situ friction measurement in an ultrahigh vacuum. MoS 2 coatings (120 nm thick) were deposited on (100) Si substrates. No trace of oxygen contamination was detected by X-ray photoelectron spectroscopy or Auger electron spectroscopy. In these conditions, reciprocating friction of the film against SiC spherical pins (normal load, 1 N; maximum hertzian pressure, 0.66 GPa; vacuum state, 50 nPa) gave extraordinary low friction coefficients below 0.005, which were often difficult to measure with the equipment available. In light of wear debris and surface analyses, the mechanisms of this superlow friction of MoS 2 are discussed.

TiN and CrN PVD coatings on electroless nickel-coated steel substrates

Abstract: Plasma-assisted (PA) PVD ceramic coating such as TiN have so far achieved only very limited use on cheap low-alloy steels, owing to problems relating to both corrosion resistance and the need for load support from the underlying material. Here we report tests to asses the wear and corrosion performance of TiN and CrN PAPVD coatings on phosphorus-doped electroless nickel (ENiP)-coated steels. It is shown that this route offers a potentially cost-effective means of utilizing PAPVD ceramic films on lower grade steels. In particular, CrN/ENiP on AISI 304 stainless steel is shown to exhibit a promising combination of wet abrasion resistance with good corrosion properties.

Plasma and ion-beam-assisted deposition of multilayers for tribological and corrosion protection

Abstract: Multilayers of Ti/TiN and Al/AlN were deposited on steel and silicon by magnetron sputtering and ion-beam-assisted deposition. Compositions and film thicknesses were determined by Rutherford backscattering. Hardness was measured with a dynamic ultramicrohardness tester. The corrosion protection potential in an aqueous environment was evaluated by electrochemical techniques. The results are discussed in terms of the structure and composition of the multilayer arrangement. It turns out that the multilayer coatings generally show a better corrosion protection performance than both the pure metal and the pure nitride films, with an optimum ratio of nitride film thickness to metal film thickness for a maximum corrosion protection effect. The hardness values are in between that of nitride single film and those of metal single films. Deposition of multilayers with the possibility of selecting the thickness ratio of metal to nitride film for particular mechanical and chemical requirements allows a controlled adaption of the film features to a given application problem.