Showing papers in "Surface & Coatings Technology in 1996"

A study of the wear mechanism of diamond-like carbon films

Abstract: In the present work, the tribological mechanism of diamond-like carbon (DLC) films was investigated. DLC films were coated on metallic substrates (M50 steel, Ti-6Al-4V alloy and AISI 440C steel) by methane ion-beam deposition. Pin-on-disc experiments showed that the DLC films possess excellent wear resistance and exhibit low values of friction coefficient (ƒ

Oxygen diffusion barrier properties of transparent oxide coatings on polymeric substrates

Abstract: Gas diffusion properties of transparent thin film coatings have been under investigation by a number of workers for application in food and medical packaging. Work on the oxygen permeation properties of high barrier thin film coatings is reviewed, with emphasis on the coating thickness dependence of the oxygen permeation rate. Comparison of the coating thickness dependence of the oxygen permeation properties of evaporated, sputtered and plasma-deposited transparent oxide and organic coatings suggests that the reduction in permeation due to the coatings is limited by transport through coating defects (e.g. pinholes, grain boundaries or microcracks). The usefulness of such measurements as a probe of coating microstructure is assessed by reviewing structural and permeation investigations of metallized coatings on polymers.

Preparation of amorphous diamond-like carbon by pulsed laser deposition: a critical review

Abstract: A critical review of the pulsed laser deposition (PLD) of amorphous diamond-like carbon (DLC) films is presented. A short review of the PLD process is followed by a review of various experimental configurations for DLC deposition and a discussion of the influence of process parameters on the composition and energy of ablated carbon plumes. Particular emphasis is given to the relationship between plume properties and film structure and mechanical characteristics. For the first time, a cumulative influence of the laser power density (fluence) and wavelength on the formation and properties of DLC films is shown. The influence of bias, additional auxiliary energy, substrate temperature, and the presence of hydrogen is also discussed. A fluence-wavelength region for DLC formation is proposed and correlated with the kinetic energy of ablated carbon species. It is shown that lower fluences are required to produce DLC films when shorter-wavelength lasers are used. The latest available results on applications of PLD DLC films as protective coatings for reducing friction and wear are also discussed. Methods are proposed to improve film adhesion to steel substrates, so that DLC films can be used in highly loaded friction contacts. Finally, process improvements that are necessary to permit scaling up PLD for growing DLC films are outlined.

Role of environment deposits and operating surface temperature in spallation of air plasma sprayed thermal barrier coatings

Abstract: Spallation of air plasma sprayed (APS) thermal barrier coatings (TBCs) was investigated on power generation combustors, military turboshaft engines, and commercial turboprop engines. In each case, irrespective of operating conditions or geographic location, spallation was linked to the presence and infiltration of high temperature molten phases of similar composition. Electron microprobe analysis found that, from all the possible oxides available in the external environment, only CaO, MgO, Al2O3 and SiO2 (CMAS) are incorporated in the molten phase that infiltrates the TBC microstructure. Fe and Ni oxides from metallic components and zirconia and yttria from the TBC were also found in varying amounts in the molten phase. The melting and recrystallization behavior of CMAS deposits was carefully defined by differential thermal analysis.

An investigation of the relationship between graphitization and frictional behavior of DLC coatings

Abstract: In our recent studies, diamond-like carbon (DLC) films were found to possess low coefficient of friction (f < 0.1) and excellent wear resistance. The reduction in f was found to be consistent with wear-induced graphitization of the DLC structure. The purpose of the present work was to study the effect of load and sliding velocity on the frictional behavior and graphitization process occurring in DLC during wear. Pin-on-disc experiments were conducted on DLC-coated SiC substrates at sliding velocities between 0.06 and 1.6 m s−1 under 1 and 10 N loading levels using ZrO2 balls as the pin material. Analytical transmission electron microscopy was used to characterize the structure and microstructure of the wear debris after testing. The results showed that both sliding velocity and contact load influence the graphitization process. Higher sliding velocities increase the contact frequency and the rate of temperature rise that may facilitate the release of hydrogen atoms from the sp3 structure. Higher loading enhances shear deformation and transformation of the weakened hydrogen-depleted DLC structure into graphite [10]. The present findings are consistent with our earlier proposed wear-induced graphitization mechanism for these films. An equation was developed to describe the transformation kinetics of DLC into graphite as a function of sliding velocity and applied stress.

A micro-abrasive wear test, with particular application to coated systems

Abstract: A test methodology is described which permits a rotating sphere test, also used to measure coating thickness, to be used as a small-scale abrasive wear test. The ability of the test to measure intrinsic wear resistance for thin coatings is demonstrated together with a new method of analysis which allows simultaneous evaluation of the wear resistance of both substrate and coating from their combined wear behaviour in a single test. The method has been applied to physically vapour deposited TiN, TiCN and TiAlN coatings on tool steel substrates, as well as to bulk samples of metals, ceramics and soda-lime glass.

Multilayer coatings for improved performance

Abstract: Although single-layer coatings are finding a range of applications in many sectors of engineering, there are an increasing number of applications where the properties of a single material are not sufficient. One way to surmount this problem is to use a multilayer coating that combines the attractive properties of several materials, each chosen to solve a problem in the application. Simple examples of this include the use of interfacial bonding layers to promote adhesion, or thin inert coatings on top of wear-resistant layers to reduce the corrosion of cutting tools. There is, however, mounting evidence that the multilayer structure produced when many alternating layers of two materials are deposited can lead to improvements in performance over a mixed coating (by virtue of the introduction of new interfaces) even if the two materials do not have specific functional requirements in the intended application. In this paper results for multilayer coatings in the TiN system will be presented showing the advantages of compositionally and structurally modulated coatings over conventional single-layer titanium nitride coatings in tribological applications. The results will be discussed in light of the microstructure and properties of the coatings produced by the two different multilayer coating techniques.

Recent advances in polymer thin films prepared by plasma polymerization Synthesis, structural characterization, properties and applications

Abstract: In this paper, the basic features of plasma polymerization of organic thin films are reviewed Principal synthesis technologies generally used precursors and new developments in the 1990s are taken into account Effects of plasma polymerization conditions on the structures and properties of the films are briefly discussed Modern analytical techniques used for characterization of plasma polymer firms, such as Fourier transform infrared and electron spectroscopy for chemical analysis, etc, are systematically reviewed Physical properties, including surface properties, permeability, electrical and optical properties, are concisely described with the emphasis on the studies in recent literature Some proposed applications of these films in mechanical, electrical and optical industry are presented

Novel thermodynamically stable and oxidation resistant superhard coating materials

Abstract: A theoretical concept for the design of novel, nanocrystalline and thermodynamically stable materials with hardness of ≥50 GPa (about 5000 kg mm−2), elastic modulus of ≥500 GPa and a high stability against oxidation in air up to 800°C is described together with its experimental verification on several systems nc-MexN/a-Si3N4 (Me  Ti, W, V). The concept is based on avoiding the formation and multiplication of dislocations in the nanocrystalline phase, and blocking the crack propagation in a 0.3–0.5 nm thin amorphous tissue. The theoretical principles of the design of such materials and the thermodynamic criteria for the segregation of the nc- and a-phases, which is necessary for the preparation of such materials, are discussed. Several micron thick films of such materials have been prepared by plasma CVD at a rate of 0.6–1 nm s−1 from the corresponding metal halides, hydrogen, nitrogen and silane at deposition temperatures of ≤550°C. A low content of chlorine of ≤0.3 at.% assures their stability against corrosion in air. Upon microindentation up to a load of ≥100 mN the films show a remarkably high elastic recovery of about 80%. Unlike diamond, c-BN, and C3N4 these materials are thermodynamically stable and relatively easy to prepare.

Physical vapor deposition tool coatings

Abstract: Physical vapor deposition (PVD) of hard coatings such as titanium nitride have been an industrial reality since the beginning of the 1980s. Two PVD processes, low voltage electron beam and cathodic arc deposition, were responsible for the early commercial success of hard coatings on high speed steel tooling. Since that time, two other PVD processes have also been prosperous in the industrial world—high voltage triode electron beam and unbalanced magnetron sputtering. There are many similarities and differences between these four PVD hard coating processes, but not all of the commonly used PVD hard coatings can be deposited well in the four systems. Titanium nitride and titanium carbonitride are the two most widely used PVD tool coatings, and they can be deposited in all four PVD systems. Titanium aluminum nitride can be deposited easily with the unbalanced magnetron process and also with the cathodic arc processes as long as cast targets are used. Uniform composition cannot be maintained with the electron beam processes because of the different vapor pressures of titanium and aluminum. New PVD hard coatings are being developed and applied. Diamond-like carbon is now being tried for some non-ferrous cutting applications, and metal-carbon films are showing promise. Polycrystalline nitride superlattice coatings made of thin alternating layers of two hard coatings such as titanium nitride and niobium nitride are showing potential for abrasive cutting situations because of the very high hardness (up to 5200 HV) of these coatings. The search continues for potentially even better tool coatings. An intense effort is underway to produce the superhard crystalline carbon nitride, which is predicted to have a hardness as hard or even harder than diamond. Similarly, cubic boron nitride (CBN) coatings are still very much in the development stage. Thin CBN coatings can now be deposited up to approximately 2000 A in thickness, but stress in the films prevents thicker films from being made. Aluminum oxide, which has been very difficult to make by PVD techniques, except in the very slow r.f. sputtering mode, can now be deposited in the d.c. magnetron mode when pulsed power is used.

Characterization of wear protective AlSiO coatings formed on Al-based alloys by micro-arc discharge treatment

Abstract: The wear life of components manufactured from Al-based alloys can be drastically increased by the application of ceramic coatings. However, coatings deposited by conventional methods such as vacuum deposition or plasma spray have either insufficient adhesion to Al-based materials or the deposition process causes the component to overheat. A recently developed micro-arc discharge oxidizing (MDO) technique allows for the formation 100–200 μm thick AlSiO coating on the surface of Al alloys. A composite Al2O3SiO2 coating is formed at room temperature as a result of a reactive process between Al in the alloy itself and O and Si supplied by an electrolyte. AlSiO coatings were investigated with XPS, Vickers and nanoindentation hardness tests, ball-on-disk, and block-on-ring friction and wear tests. Coatings were found to consist of at least two phases: a hard Al2O3 phase and a softer aluminasilicate phase. A maximum hardness of 17 GPa was found for coatings with highest content of Al2O3 phase. The tribological properties of AlSiO coatings with different composition are discussed. The lowest friction coefficient was found for the Al0.26Si0.08O0.66 coating and was measured around 0.15–0.25 depending on the test environment. The application of this coating decreased the wear rate of components fabricated from an Al-based alloy by several orders of magnitude and permitted operation of coated friction pairs at 1 GPa contact load.

The oxidation behaviour of sprayed MCrAlY coatings

Abstract: Turbine blades are protected against high temperature oxidation by thermal barrier coating (TBC) systems, which consist of a ceramic top coating (ZrO 2 /Y 2 O 3 ) and a metal bond coating (MCrAlY, M  Ni, Co). At high temperatures and under oxidative conditions, between the MCrAlY and the ceramic top coating an oxide scale is formed, which protects the metal against further oxidation. The oxidation behaviour of the thermally sprayed MCrAlY is influenced by the coating process and the composition of the metal alloys. This work is concerned with the isothermal oxidation behaviour of vacuum plasma sprayed (VPS) MCrAlY coatings. The MCrAlY powders used have different aluminium contents: 8 and 12 wt.%. The MCrAlY specimens are oxidized at 1050 °C in air as well as in helium with 1% O 2 and the oxidation kinetics are determined thermogravimetrically. The microstructure, morphology and thickness of the oxide scales formed are characterized by metallography, SEM, TEM and XRD. After short time oxidation (6 h) θ-Al 2 O 3 is the main constituent of the oxide scale. Exposure times of 500 h and more lead to oxide scales consisting of α-Al 2 O 3 . Moreover, after a long time oxidation, Cr 2 O 3 and CoO (CoO on the coatings with 8 wt.% Al) are formed. The oxidation rates of both MCrAlY coatings are the same. Beneath the oxide scale an Al-depleted zone is formed and this zone is considerably thicker with the coating with 8 wt.% Al, because the amount of β-NiAl phase in this coating is lower than that in the coating with 12 wt.% Al. The oxide scale formed in He—1% O 2 consists of α-Al 2 O 3 and Cr 2 O 3 on both MCrAlY coatings.

Plasma immersion ion implantation of stainless steel: austenitic stainless steel in comparison to austenitic-ferritic stainless steel

Abstract: It has been shown previously in the literature that plasma immersion ion implantation (PHI) can increase the wear resistance of austenitic stainless steel without losing its corrosion resistance. In this work, the effect of PHI treatment on the microstructure and the properties of an austenitic (X6CrNiTi1810, AISI 321) and a duplex austenitic-ferritic (X2CrNiMoN2253, AISI 318) stainless steel has been studied and the results compared. Three different treatment temperatures and treatment times were used. The microstructures were studied by optical metallography and glancing angle X-ray diffraction (XRD). The formation of expanded austenite was observed in both steels up to treatment temperatures of 400 °C. The ferrite in the duplex austenitic-ferritic steel was also transformed to expanded austenite. At 500 °C, a surface layer consisting of CrN was formed on the duplex austenitic-ferritic steel whereas the modified layer on the austenitic steel was still expanded austenite with a small amount of CrN precipitation. Elemental depth profiling by sputtered neutral mass spectrometry (SNMS) revealed a similar treatment depth for both materials up to 400 °C, which was a function of treatment temperature and time. A pin on disc tribometer was used to determine the tribological behaviour. A change in the wear behaviour was observed and the wear depth decreased relative to untreated material. This was due to an increase in the surface hardness and a decrease in the coefficient of friction. The decrease in wear depth correlated with the thickness of the modified layer. The best results were found with the duplex austenitic-ferritic steel at a treatment temperature of 500 °C and can be attributed to the formation of a CrN layer. Corrosion tests have shown that good corrosion resistance was preserved up to 400 °C for both materials with only a small decrease being observed. This is due to nitrogen remaining in solid solution without CrN-precipitation. At a treatment temperature of 500 °C, the corrosion resistance decreased dramatically, especially for the duplex austenitic-ferritic steel where a layer of CrN was formed. These results show the capability of PIII treatment to increase the wear resistance of these stainless steels without losing their good corrosion performance. This may allow the use of such steels in applications where the poor wear resistance of the untreated material would normally prohibit their use. In comparison to the austenitic steel, the duplex austenitic-ferritic steel performed better after PIII treatment. For an optimum surface treatment, it is necessary to consider the substrate material as well as the treatment parameters.

Practical measurement of the residual stress in coatings

Abstract: The method adopted to measure residual stress depends on the degree of detailed information needed, the size of the part, the coating thickness and the costs which can be incurred. Three of the main approaches used at the present time are discussed here. These are: first, the hole drilling method which returns a value of the macrostress and an indication of any anisotropy in coatings of thickness greater than about 0.3 mm; second, the cantilever beam methods applicable to thinner coatings on thin substrates; third, the X-ray diffraction methods applicable to crystalline coatings of a thickness which can be penetrated by the X-ray beam. This last method can be sub-divided into two groups where, in the first, the standard Bragg-Brentano diffractometer available in most laboratories is used, and, second, the more recent glancing incidence methods in which the stress in thin coatings or the surface of thicker coatings can be studied at depths of as little as 1 μm.

Influence of substrate roughness and temperature on the adhesion/cohesion of alumina coatings

Abstract: This article is devoted to the study of the adhesion/cohesion (A/C) of alumina coatings sprayed on four different substrates: aluminium alloy, titanium alloy, cast iron and mild steel. Fused and crushed alumina particles (−45 + 22 μm) were sprayed with a custom-made torch with a nozzle of internal diameter 7 mm working with an arc current of 600 A and a plasma gas mixture of 45 slm Ar, 15 slm H 2 , which resulted in a voltage of 64 V and a thermal efficiency of 52%. The particles were injected internally and were almost fully molten (less than 3% α-phase in the collected powders after their passage in the plasma jet). The influence of substrate roughness and temperature before (preheating), during and after spraying was systematically studied. R a values were 6–8, 10–13 and 14–21 μm and the substrates were preheated to temperatures T p of 170, 200, 300, 350, 380 and 500 °C. The most important results were the following: 1. • for cold substrates A/C increased with R a up to a maximum of 20 MPa 2. • for substrates whose expansion mismatch with alumina was less than 4.10 −6 K, the highest A/C values (50–60 MPa) were obtained with T p between 350 and 500 °C provided the substrates were not oxidized 3. • at these temperatures the highest values of A/C were obtained with the lowest R a 4. • when particle velocity and surface temperature upon impact decreased, for example by load effect, A/C values decreased too

Recent progress in filtered vacuum arc deposition

Abstract: During this decade significant advances have been made both in the understanding and implementation of filtered vacuum are deposition. Rigid rotor models have been analyzed statistically, and new models which treat the mutual influence of the electrons and ions on each other self-consistently, take into account the centrifugal force on the ions, and take into consideration collisions, have been formulated. It was shown that the plasma transport efficiency is limited by drifts caused by the centrifugal force and by the electric field generated by charge separation in the plasma. For a range of magnetic fields strengths for which the ions are not magnetized, i.e., confined to a Larmor radius less than the duct radius, the transport efficiency for Cu plasma is about 10%, and depends only weakly on the magnetic field strength. Increased transmission is found when the ions are magnetized, reaching about 50% for a 36–60 mT field in typical configurations. The plasma transport efficiency and spatial distribution has been measured over a large parameter range, and correlated with the various theories. The plasma beam may be approximated as a Gaussian distribution which is displaced in the B × G direction, where G is in the direction of the centrifugal force, while a displacement in the plane of symmetry is surprisingly found in the − G direction. The total convected ion current decreases exponentially with distance from the toroidal filter entrance. Macroparticle transport within the magnetic filter has been analyzed, and it has been shown that electrostatic reflection from the walls can occur if the magnetic field is weak. Filtered arc sources with improved throughput performance and novel geometries have been built, and are now available commercially. The range of coatings deposited with FVAD has been expanded to include metals, oxides, and nitrides, as well as diamond-like carbon. In several cases, coatings having the highest quality reported in the literature have been fabricated with the FVAD technique, and one commercial application has been reported.

A comparative study of beam ion implantation, plasma ion implantation and nitriding of AISI 304 stainless steel

Abstract: This paper presents the results of a comparative study using beam ion implantation (BII), plasma ion implantation (PII), ion nitriding and gas nitriding of AISI 304 stainless steel We have demonstrated that under controlled conditions (the same treatment times of 30 and 60 min, and the same treatment temperature of 400 °C), the microstructures produced by all four techniques are similar, being mainly the formation of nitrogen in solid solution (γ N phase) However, the concentrations of nitrogen and the detectable depths of the nitrogen-enriched layers are significantly different, depending on the process Both BII and PII produce thick nitrogen-enriched layers (greater than 1 μm) at high concentrations (20–30 at%) compared with either ion nitriding or gas nitriding (layers less than 1 μm thick with low nitrogen concentrations) As a result, the load-bearing capacity after either BII or PII is much greater than after either ion or gas nitriding It has also been found that high current density implantation is crucial for the formation of the thick N-enriched layers

Effect of the substrate temperature on the structure and properties of Al2O3 layers reactively deposited by pulsed magnetron sputtering

Abstract: Alumina coatings were reactively deposited on steel substrates by pulsed magnetron sputtering at substrate temperatures (Ts) of 330–760 °C. Investigations into the structure and morphology of the layers were made via XRD and SEM techniques, respectively. As to the layer properties, the hardness was determined by nanoindentation, and the residual stresses were derived from the bending of the coated substrates. At substrate temperatures of less than 330 °C the Al2O3 layers are amorphous to X-rays, whereas γ-Al2O3 is detected at a substrate temperature Ts ≈ 480 °C. A further increase in substrate temperature to 560 °C results in the formation of a pronounced texture of γ-Al2O3. A phase mixture of textured γ- and α-Al2O3 is deposited at Ts ≈ 690 °C. At Ts ≈ 760 °C the layer consists completely of α-Al2O3 with crystallite sizes of about 1 μm. The occurrence of the crystalline γ phase at 480 °C is linked with a pronounced increase in hardness from 10 to 19 GPa. The layer hardness of pure α-Al2O3 amounts to 22 GPa and corresponds to the hardness of the bulk material.

Formation of cubic-A1N in TiN/A1N superlattice

Abstract: Ceramic superlattice is one of approaches for new ceramic film development. We deposited TiN/A1N superlattice films by arc ion-plating process, and evaluated the crystal structure of these films by transmission electron diffraction and X-ray diffraction. It was revealed that the crystal structure of A1N changes from Wurtzite type to NaCl type for superlattice periods ≤ 3 nm. The film hardness increased to about twice that of the TiN single layer film at a period = 2.5 nm. NaCl phase of A1N is one of the phases that exists under high pressure and it is expected that there will be a high bulk modulus.

Low energy, high current density ion implantation of materials at elevated temperatures for tribological applications

Abstract: Low energy, high current density ion implantation at elevated temperatures has been shown to improve significantly the tribological properties of various materials. This paper summarizes the results published previously in this research area and presents some new results. Comparisons of this technique are made with ion nitriding and high energy ion implantation conducted under similar conditions (treatment temperature, treatment time and so on) on austenitic stainless steel and tool steel materials. The microstructural analyses and tribological evaluations presented here show that all three techniques generate almost identical microstructures on each metal studied, but low energy ion implantation produces treated layers with higher nitrogen concentrations and deeper diffusion, leading to higher wear resistance. A physical model is proposed to explore the mechanisms for these advantageous phenomena. The analysis suggests that a high current density is the primary mechanism responsible for the formation of deep nitrogen-containing layers. The ion energy is of secondary importance, as long as it is sufficiently high to overcome certain surface barrier potentials, to allow the removal of native oxide layers, to prevent surface oxidation and to allow the build-up of a high concentration of atomic nitrogen on the top of the treated surface to facilitate subsequent fast diffusion. Some applications and limitations of this technique are also addressed. It seems evident that low energy implantation (slightly higher than for ion nitriding, but much lower than for high energy ion implantation) at high current densities (much higher than those used in both ion nitriding and high energy implantation) generates superior nitrogen-containing layers on many materials, and hence the superior tribological performance compared to the other two techniques.

Surface thermal cracking of thermal barrier coatings owing to stress relaxation: zirconia vs. mullite

Abstract: The initiation of surface cracks in zirconia based multilayer thermal barrier coatings is related to stress relaxation which occurs at the top surface of the coating at high temperatures. An analytical model is used to show that the compressive stresses decrease over time and become tensile upon cooling of the coating, thus resulting in cracks. Mullite, which exhibits significantly reduced stress relaxation behavior, is shown to remain in compression, thus preventing the initiation of surface cracks. This behavior is compared for the cases of equal heat flux and equal surface temperature conditions.

Plasma nitriding of Ti6Al4V alloy to improve some tribological properties

Abstract: The present study was conducted to investigate the tribological and mechanical properties of plasma-nitrided Ti6Al4V alloy. Specimens were nitrided in an H2N2 (1:8 ratio) plasma. The nitrogen concentration along the nitrided zone was obtained using the nuclear reaction analysis technique. The workpiece temperature was varied from 450 to 520 °C during the nitriding process. Pin-on-disc wear tests were carried out to evaluate the wear properties of the resultant samples and a ball-on-disc experiment was conducted to measure the friction coefficient. Microhardness tests, Scanning electron microscopy and X-ray diffraction were carried out to investigate the phases developed in the nitrided zone. It was found that the wear resistance improved considerably after the nitriding process. Three distinct layers were identified: (i) an inner layer where δ-TiN + e-Ti2N phases formed, (ii) an intermediate layer where α-(TiN) with or without e phase developed and (iii) an outer layer where precipitations were dominant.

Development and environmental durability of mullite and mullite/YSZ dual layer coatings for SiC and Si3N4 ceramics

Abstract: Development of refractory oxide environmental coatings for SiC and Si3N4 ceramics, interaction with simulated combustion environments, and long-term durability under thermal cycling in air are discussed. Mullite coating performance data indicate its ability to limit the molten salt corrosion. Two-layer mullite/YSZ coating showed promise as a barrier coating limiting enhanced silica volatilization by water vapor in a simulated lean-burn combustion environment. The mullite/YSZ dual layer coatings showed excellent resistance to cracking and debonding for 1000 h using 2-h thermal cycles in air at 1200–1300°C. A CVD SiC bond coat developed for Si3N4 dramatically enhanced the adherence of mullite coatings. Contamination of silica scale by alkali and alkali earth metal oxides resulted in enhanced oxidation and pore formation which is an issue for the long-term environmental durability of these coatings.

Tribology of naturally occurring boric acid films on boron carbide

Abstract: In this paper, we describe the formation and self-lubricating mechanisms of naturally occurring boric acid films on boron carbide (B 4 C) substrates. The sliding friction coefficients of yttria/partially stabilized zirconia pins against plain B 4 C substrates are quite high at 0.3–0.4, but are 6–10 times lower against the B 4 C substrates subjected to annealing at 800°C. We determined that this low friction was the result of a thin boric acid film that forms naturally on the sliding surface. During annealing at 800°C, the exposed surface of B 4 C undergoes oxidation and forms a layer of boron oxide. During cooling, this layer undergoes a secondary chemical reaction with moisture in the air to form a thin boric acid film that is responsible for the ultralow friction coefficients reported here. As reported in previous literature, the low-friction mechanism of boric acid is associated with its layered-triclinic-crystal structure. The atoms on each layer are closely packed and strongly bonded to each other but the layers are widely separated and are held together by van der Waals forces. During sliding, these atomic layers can align themselves parallel to the direction of relative motion and slide over one another with relative ease to provide the 0.03–0.05 friction coefficients reported here. Raman spectroscopy was used to elucidate the chemical bond structure of bulk boron carbide and lubricious boric acid film on sliding surfaces.

Surface treatment of biomaterials by ion beam processes

Abstract: Surface treatment of biomaterials is becoming an increasingly popular method of improving device function and biocompatibility without the expense and time required to develop new materials. Ion-beam-based processes, such as ion implantation and ionbeam-assisted deposition (IBAD), have proven particularly successful in this area because they offer a wide array of beneficial surface property modifications without adversely affecting bulk properties. For instance, orthopedic prostheses are made harder and more wear resistant by ion implanting their articulating surfaces. Many other devices, such as orthodontic appliances, surgical instruments and venous catheters, are treated to improve friction, fretting resistance and biocompatibility. IBAD is used to apply antimicrobial coatings to catheters and other devices, to apply sealant coatings to prevent oxygen or water vapor permeation, and to create metallized traces on polymers for use in flexible circuitry. This paper reviews the major medical device applications of ion beam-based technologies.

Improvement of the wear resistance of 316L stainless steel by laser surface alloying

Abstract: In order to improve the sliding wear resistance of stainless steel components, carbide-hardened surfaces were produced on AISI 316L austenitic stainless steel by laser surface alloying. Different powdered precursors (Cr3C2, a mixture of Cr3C2 and Cr and a mixture of Ti and SiC) were applied to the steel surface and irradiated using a continuous wave, 300 W, Nd-YAG laser. The surface alloys resulting from the incorporation of Cr3C2 alone or mixed with chromium were composed of austenite (y) dendrites surrounded by a γ-M7C3 eutectic (M≡Fe or Cr); their microhardness ranged from 380 to 450 HV. Superficial hardening by titanium carbide was obtained using a mixture of Ti and SiC powders as precursors; Ti and SiC particles dissolved in the melted pool, leading to the precipitation of fine TiC particles during solidification. The microhardness of the surface alloy was about 350 HV. For both chromium carbide and titanium carbide surface alloying treatments, the processing conditions were determined which led to a noticeable improvement of the sliding wear resistance of AISI 316L stainless steel without any detrimental effect on its high temperature oxidation resistance.

The replacement of electroplating

Abstract: With the increasing concern over toxic wastes produced by conventional metal finishing operations there is a strong drive in the USA to replace “dirty” electroplating processes (especially chrome and cadmium) with “clean” technologies. While many companies and military agencies continue to think in terms of less polluting electroplates, others are turning away from bath technologies completely, in favor of the modern high performance dry coating methods such as physical vapor deposition (PVD), chemical vapor deposition (CVD) and thermal spraying. Under funding from the US Defense Department's Advanced Research Projects Agency, various dry alternatives to electrolytic hard chrome, namely PVD, plasma nitriding, high velocity oxyfuel (HVOF), laser CVD and laser cladding, have been examined for use both in original equipment manufacture and in rebuilding worn components. In this paper hard chrome replacement is used to illustrate the requirements for replacement of electrolytic coatings in general. Hard chrome alternatives are being evaluated to ensure not only that their performance is at least comparable with the chrome they replace, but also that they fit with the way in which the coated components are produced, used, and refurbished. They must be reliable and cost-effective, and must fit the needs of the end user over the entire life cycle of the coated component. HVOF, PVD and duplex plasma-nitride PVD coatings show great promise as replacements in a variety of applications, from bearing surfaces and hydraulics to decorative finishes. The combination of performance data with cost analyses shows that these alternatives can be cost-effective chrome replacements. For example, HVOF coatings can provide more than twice the life in sliding wear at half the cost.

Advanced solid lubricant coatings for high vacuum environments

Abstract: Solid lubricant coatings for vacuum applications have seen considerable developments for many years, because of the use of advanced coating techniques, such as physical or chemical vapor deposition processes. The need for fully understanding the relationships between the nature of the lubricant coatings and their tribological performances in relation to the nature of the environment during sliding has become more pressing. The present paper discusses and compares the friction behavior of two kinds of thin film solid lubricant (pure MoS 2 and hydrogenated diamond-like carbon (DLC)) from a pressure range less than 5×10 −8 hPa to ambient air. Friction coefficient values less than 0.15 in ambient air and less than 10 −2 in ultrahigh vacuum have been recorded using pin-on-fiat tribometers. Present results are also compared with previous work published by others. The potentiality of DLC coatings used as solid lubricant for space applications is thus highlighted, in comparison with the more extensively used MoS 2 coatings.

Effects of boric acid on the electrodeposition of iron, nickel and iron-nickel

Abstract: Iron (Fe), nickel (Ni), and iron-nickel (FeNi) platings were carried out potentiostatically on rotating disk electrodes The effects of boric acid on the iron and nickel reduction rates were evaluated Experimental results support the surface competition of adsorption of ferrous and nickel ions on the electrode surface in FeNi alloy deposition Boric acid prevents the electrode surface passivation on nickel reduction Furthermore, boric acid acts as a surface agent and functions as a selective membrane which blocks the passage of the reduction of nickel but permits the reduction of iron in a retarded rate The existence of boric acid alters the polarization behaviours in the FeNi system significantly

Wear performance and mechanism of electroless NiP coating

Abstract: An electroless NiP coating has been deposited onto an AISI 1020 plain carbon steel by using an acid bath based on NiCl 2 , as source of nickel cations. The tribological behavior under dry non-lubricated conditions has been evaluated by employing the pin-on-disc test, where the pin was made of AISI 52100 steel. The effects of the heat treatment carried out at 400 and 260°C for 1 and 25 h, respectively on the hardness, friction and wear behavior of the coatings were investigated and compared against the properties of the unplated and as-deposited samples. Both optical and electron microscopic techniques were used to study the wear surface. It was shown that the wear resistance was greatly increased with both heat treatments, the treatment performed at 400°C being a little more effective, although no influence was observed on the frictional coefficient.