Showing papers in "Surface & Coatings Technology in 1995"

Multilayer PVD coatings for wear protection

Abstract: The PVD technology is well suited to realize new and advanced coating concepts like gradient coatings, metastable coatings, multicomponent coatings, multilayer or superlattice coatings. Among these coating concepts the multilayer coatings seem to be the most promising concept because it meets many requirements (e.g. multifunctional character, moderate residual stresses, good adherence to metallic substrates, proper hardness to toughness ratio and low friction coefficients) for a composite exposed to complex wear conditions. Further the concept allows metastable and multicomponent materials to be introduced in a graded multilayer arrangement. By this way different layer concepts can be realized simultaneously. A functional and a structural design, taking into account the material selection for the individual layers, the adjustment of the interface volume and constitution and the optimization of the individual layer sequence and thickness, allows a tailoring of properties and performance. Covalent or ionic materials like B 4 C, SiC, Si 3 N 4 or Al 2 O 3 can be introduced as intermediate layers in the multilayer arrangement, raising hardness and high temperature strength without reducing adherence or toughness of the coating. New results concerning constitution, properties and application characteristics for multilayer coatings mainly based on TiC/TiN are presented.

Structure and properties of plasma-nitrided stainless steel

Abstract: A series of plasma-nitriding experiments has been conducted on AISI 304L austenitic stainless steel at temperatures ranging from 400 to 600 °C using a pulsed d.c. plasma with various pulse duration/repetition ratios in an N2H2 gas mixture. The structure and composition of the plasma-nitrided surface layer were analysed by means of X-ray diffraction (XRD), scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), optical microscopy and microhardness testing. The corrosion behaviour of the S phase was also investigated. The maximum Knoop hardness after plasma nitriding is about 1400 HK 0.01 and the maximum thickness of the compound layer is 34 μm. XRD patterns show that the surface layer consists of the S phase only. The corrosion performance was tested in 0.05 M Na2SO4 solution at pH 3.3 and in neutral 3.5% NaCl solution at ambient temperature. Potentiostatic and potentiodynamic experiments yielded slightly higher passive corrosion currents for plasma-nitrided 304L. Pitting corrosion in neutral electrolytes containing chloride ions was observed only for untreated 304L. The passive layers formed on both types of samples were similar in constitution and thickness as determined from XPS sputter profiles.

Residual stresses in PVD hard coatings

Abstract: High macroscopic residual stresses in PVD hard coatings up to about −10 GPa are caused by both the thermoelastic effects and grown-in defects, generated by fast particles during deposition. Defect recovery, relaxation by plastic deformation or cracking limit the residual stresses. The mechanical behaviour of coated materials can be explained by the combination of the residual stresses and the exterior stresses (bending, hardness, critical load, erosion, abrasive wear). Owing to the lack of generalized knowledge about these relations a prediction of the performance of tools or components has to be sketchy.

Thick ceramic coatings using a sol gel based ceramic-ceramic 0–3 composite

Abstract: A sol gel based coating technology has been developed for making high quality, thick ceramic coatings. The films are ceramic-ceramic 0–3 composites made by dispersing ceramic powders in a sol gel solution. The resulting system has all the benefits of sol gel, i.e. ease of fabrication, ability to coat complex geometries and relative cost effectiveness, but is not limited in film thickness. Lead zirconate titanate (PZT), yttria- and ceria-stabilized zirconia, titania, silica and alumina films have been fabricated in the 5–200 μm thickness range. The electrical properties of the PZT films are comparable with those of the bulk ceramic, and the zirconia and alumina films exhibit excellent potential as corrosion- and wear-resistant coatings as well as thermal barrier and performance enhancing coatings.

Progress in the analysis of the mechanisms of ion nitriding

Abstract: In this paper the mechanisms of ion nitriding are discussed, particular attention being paid to d.c. diode nitriding. Recent trends in the analysis of the mechanisms of d.c. diode nitriding are reviewed, emphasizing the importance of the cathode fall region. Diagnostics of active species and calculation of their densities in the plasma are presented and related to modelling. The hydrogen effect and plasma-solid interaction are also discussed. New developments and alternatives to d.c. diode nitriding such as triode nitriding are highlighted.

Tribological characteristics of DLC films and duplex plasma nitriding/DLC coating treatments

Abstract: An innovative approach to improving tribological behavior of surfaces and meeting long-term durability requirements of engineering devices is to design and develop novel systems incorporating multilayers and/or duplex diffusion/plasma coating treatments. In the present work, the wear and friction characteristics of diamond-like carbon (DLC) films and composite surface layers were studied by conducting pin-on-disc experiments. M 50 steel and Ti-6A1-4V alloy were used as substrate materials. The composite layers consisted of a N-diffusion zone obtained by ion nitriding, followed by a 500 A vapor-deposited Si bond layer and a 0.4 µm DLC film. The purpose of the bond layer was to enhance adhesion between the substrate and the DLC films. An ion-beam method was used for the deposition of the DLC films. The pin-on-disc results showed that for both materials the DLC coating produced a reduction in the coefficient of friction of about one order of magnitude. The reduction in the coefficient of friction was found to be consistent with the formation of a carbon-rich transfer film on the contact surfaces. Wear scar profiling and weight loss calculations showed that the wear resistance of the DLC-coated materials was dramatically improved. Comparisons between duplex N-diffusion layer/DLC coating and single DLC coating on Ti-6A1-4V alloy substrates showed that the duplex treatments possessed a significantly higher wear resistance. Nitriding was found to cause substrate hardening that reduces subsurface deformation, thus improving coating support and extending considerably DLC film lifetime.

Application-oriented modifications of deposition processes for diamond-like-carbon-based coatings

Abstract: Hydrogenated amorphous carbon films (a-C:H) exhibit some “diamond-like” properties as high wear resistance and low electrical conductivity. However, to fulfil requirements for special applications, e.g. high wear resistance combined with high electrical conductivity or low surface energy, the films had to be modified. These modifications can be realized by the incorporation of other elements (metals, F, O, N or Si) into the a-C:H network structure either by co-sputtering of metals or by adding adequate reactive gases to the working gas. It was found that the surface energy of a-C:H films can be definitely changed by the incorporation of elements such as F, N, O or Si into the films, using a mixture of acetylene with, for example, fluorinated hydrocarbons, nitrogen or silicon-based hydrocarbons as a working gas. Hard coatings with decreased or increased wetting angles against water were prepared. For example a Teflon ® -like material with low adhesion forces could be deposited, while the tribological properties of this coating are similar to those of diamond-like carbon coatings. Metal-containing amorphous hydrocarbon (M-C:H) films with 15 at.% metal (e.g. Ta, W, Ti or Nb) or less have nearly the same wear resistance and friction coefficient as a-C:H, but their electrical conductivities are several orders higher. The variation in the metal concentration in such coatings has to be kept under good control to increase the adhesion forces to the substrate material by the use of metallic interface layers. We have developed a reactive d.c. magnetron sputtering technique for the preparation of M-C:H films with well-defined metal contents in the graduated interface and in the functional top coat respectively. A reproducible preparation of coatings with desired metal contents was achieved for different industrial components.

Nitriding of austenitic stainless steel by plasma immersion ion implantation

Abstract: Plasma immersion ion implantation (PI3™), in which the diffusion of nitrogen from a low pressure r.f. plasma is combined with the implantation of nitrogen ions at energies up to 45 kV, is an effective means of nitriding austenitic stainless steel. At temperatures up to 450 °C, tribological properties can be improved without loss of corrosion resistance. In common with other nitriding processes in this temperature range, a supersaturated f.c.c. phase is formed, sometimes described as expanded austenite, which is maintained to very high nitrogen concentrations. At higher temperatures, chromium nitride is precipitated and the expanded austenite decomposes, leading to a reduction in corrosion resistance. Glancing-angle X-ray diffraction (XRD) of PI3-treated AISI 316 stainless steel at temperatures between 350 and 450 °C suggests that a highly homogeneous layer of expanded austenite is produced. The expansion increases with increasing process time, but decomposition of the supersaturated phase occurs after several hours of treatment if the temperature is too close to 450 °C. For a fixed process time, the expansion appears to be greatest at the lower temperatures (350 °C), although it can also be influenced by other processing parameters such as plasma density. Microstructural examination by cross-sectional transmission electron microscopy (TEM) has challenged the identification of the supersaturated phase as expanded austenite and reveals the complexity of the modified layer not seen by glancing-angle XRD. Most striking is the formation of a thick (2–3 μm) amorphous zone which may contain nanocrystalline precipitates of CrN and α-ferrite. A highly defective layer (up to 2 μm thick) of expanded austenite has been observed to underlie the amorphous zone where nitrogen diffusion is facilitated by the high defect density. Only partial reconciliation of the TEM results with the XRD observations has been possible to date.

XPS in the study of high-temperature oxidation of CrN and TiN hard coatings

Abstract: The oxidation behaviour of the CrN and TiN hard coatings prepared by reactive sputtering at 200 °C was studied using X-ray photoelectron spectroscopy. The formation of thin surface overlayers on top of nitride coatings was observed even at room temperature. At elevated temperatures the mechanism of nitride oxidation proceeds by a progressive displacement of nitrogen by oxygen. At sufficiently high temperatures a tendency towards phase separation between the nitride and oxide is observed, resulting in the formation of Cr 2 O 3 and TiO 2 layers, respectively.

Characterization of transfer layers on steel surfaces sliding against diamond-like hydrocarbon films in dry nitrogen

Abstract: Carbon-rich transfer layers on sliding contact surfaces play important roles in the tribological performance of diamond-like hydrocarbon (DLHC) films. In this study, we investigated the nature of these layers formed on M50 balls during sliding against DLHC films (1.5 μm thick) prepared by ion-beam deposition. Long-duration sliding tests were performed with steel balls sliding against the DLHC coatings in dry nitrogen at room temperature, approximately 22 ± 1 °C. Results indicated that the friction coefficients of test pairs were initially about 0.12 but decreased steadily with sliding distance to 0.02–0.03 and remained constant throughout the tests, which lasted for more than 250 000 sliding cycles (approximately 30 km). This low-friction regime appeared to coincide with the formation of a carbon-rich transfer layer on the sliding surfaces of M50 balls. Micro-laser Raman spectroscopy and electron microscopy were used to elucidate the structure and chemistry of these transfer layers and to reveal their possible role in the wear and friction behavior of DLHC-coated surfaces.

Friction and wear behavior of titanium nitride, zirconium nitride and chromium nitride coatings at elevated temperatures

Abstract: Friction and wear behavior of TiN, ZrN and CrN coatings in contact with Inconel (trademark of the International Nickel Co, Inc) 718 were investigated using a ring-on-disc test machine at 500°C and 600°C The applied load was 267 N and the linear velocity was 124 m s−1 The wear of the coatings was extremely small compared with that of Inconel 718, which was found to depend strongly on its mating coating At 500°C, the wear performance of ZrN- and CrN-Inconel 718 wear couples were comparable to or better than an uncoated couple, respectively At 600°C, the wear performance of CrN-Inconel 718 was markedly better than that of ZrN-Inconel 718 The microstructural and chemical characterization of wear surfaces and debris were investigated by scanning electron microscopy and energy dispersive spectrometry X-ray diffraction revealed that the wear debris from all wear couples consisted of NiCr2O4, NiO, Cr2O3 and Inconel 718 Based on the morphology and chemistry changes of wear surfaces and phase compositions of wear debris, the adhesive and oxidative wear was the primary mechanism for the uncoated Inconel 718 wear couple For the coating-Inconel 718 couples, the abrasive and oxidative wear accounted for material removal of the Inconel 718 disc; the adhesive wear and oxidative and chemical wear were responsible for the coating removal

Examination of mechanical properties and failure mechanisms of TiN and Ti−TiN multilayer coatings

Abstract: A number of studies have been carried out to establish mechanical properties of single and multilayer hard coatings. However, the mechanisms of deformation, cracking and delamination of coatings under ploughing and shear stress are not fully understood. A fractured cross-sectional specimen preparation technique through hardness indentation and scratch tests on hard coatings has been used in conjunction with high resolution SEM to observe deformation and fracture behaviour occurring as a result of these tests. TiN and Ti−TiN multilayer coatings were deposited on M2 high speed steel and silicon substrates using an unbalanced magnetron sputtering system. Hardness measurements and scratch tests were performed to monitor the mechanical properties. X-ray diffraction was used for phase identification. Coatings comprising fine columnar TiN behaved like closely congregated strong fibres: they were found to accommodate a large amount of ploughing and shear stress through densification and shear deformation. On increasing the load above a certain value, rupture of heavily deformed TiN initiated at defect locations and the cracks propagated and coalesced into macrocracks. When the applied load was increased to near the critical load, close packed columns separated from each other and detached from the substrate, resulting in total failure. For Ti−TiN multilayers, hardness and critical load are related to the different monolayer thickness of the Ti and TiN. The Ti layers dissipate most of the energy by means of shear deformation during the scratch test. At higher scratch loads, cracks occurred at Ti−TiN interfaces or at multilayer- substrate interfaces depending on the relative interface strengths. The influences of substrate hardness on the indentation crack pattern and scratch failure mechanism are also briefly covered in this paper.

Today's applications and future developments of coatings for drills and rotating cutting tools

Abstract: According to the sold value, 40% of all cutting tools are coated in industry today. This paper summarizes the most important application field of conventional and new coatings for cutting tools. There are two opposing development trends in the cutting industry today: (1) in dry machining the user wants to work without coolant to save the environment and production costs; (2) in high speed machining (HSC) the cutting parameters can be increased by a factor of 4–10. Aside from increasing productivity, multi-spindle heads can be replaced by one spindle, increasing the flexibility enormously. With both technologies much greater heat is produced than under normal cutting conditions. The cutting materials must have a high warm hardness and cannot work without a good heat isolation coating between the hot chips and the tool body. This paper gives an overview of the latest industrial applications of different coatings for dry and HSC cutting and characterizes the most important requirements of future coatings for cutting tools.

Hybrid techniques in surface engineering

Abstract: The potential of the newer coating methods, such as plasma assisted physical vapour deposition (PAPVD) will not be realized across a wide applications range until certain intrinsic deficiencies are overcome. These relate, for example, to processing economics and the ability to coat low cost substrates. Within the solution of the latter requirement comes the need to provide load support for the coating. Also for many applications, especially those in which abrasion and corrosion may occur, thicker films are needed. In this paper we highlight four hybrid processes which, in different ways, seek to remove these constraints of the PAPVD process and thereby considerably widen its applicability.

Laser cladding using the powder blowing technique

Abstract: A series of laser-clad samples have been produced using a 5 kW CO 2 laser and a powder blowing technique. With the correct conditions, a fused bead of alloy up to 5 mm wide and 1 mm thick can be formed on a surface. A wide variety of combinations of base plate and clad materials have been used, including base plates of C-Mn steel, SG cast iron and 304L stainless steel and powders made from Stellite 6, Eutrolloy 16262, Cenium Z20, tool steel, chromium white cast iron and bearing-metal alloys. With the correct process parameters the surface layer formed can be very uniform, with no porosity and little dilution by the base plate. The HAZ is small and the interface between the cladding and the substrate is very sharp, with little diffusion of the clad alloy into the base plate. This may be attributed to the very rapid heating and cooling rates produced by the laser irradiation, which also contributes to the production of a very fine as-cast microstructure.

The high speed sliding wear behaviour of boronized medium carbon steel

Abstract: Steels provided with boronized diffusion coatings are known to exhibit excellent wear resistance at low sliding speeds However, the sliding wear behaviour of boronized coatings at high sliding speed is not known In the present study, the high speed sliding wear and friction behaviour of a boronized medium carbon steel has been characterized using a friction dynamometer The boronized samples were tested against a hardened tool steel disc The sliding tests were carried out at velocities up to 8 m s −1 and normal loads up to 2 MPa These tests indicated that the wear rate of the steel can be reduced by about an order of magnitude as a result of boronizing The coefficient of friction of the boronized steel was higher than that of the unboronized steels especially at higher sliding speeds

Residual stresses in nitride hard coatings prepared by magnetron sputtering and arc evaporation

Abstract: Nitride hard coatings such as TiN or (Ti,Al)N produced by magnetron sputtering and arc evaporation processes are characterized by large compressive stresses up to −15 GPa. In the case of magnetron sputtering the highest stress values can be observed at high bias voltages and low substrate temperatures. Coatings produced by arc evaporation, exhibit high compressive stresses even in the case of no bias voltage. The stresses are also influenced by substrate temperatures. In both cases the growing layer is bombarded with accelerated particles. However, there are differences in the kinds of particles. We assume that in the case of sputtering Ar ions and in the case of arc evaporation metal ions with high kinetic energy push nitrogen atoms from regular lattice sites into tetrahedral holes of the rock salt type lattice and occupy the previous nitrogen sites. During arc evaporation the formation of vacancies (Frenkel defects) is also possible. Both effects, the incorporation of argon or metal atoms on nitrogen sites and the nitrogen interstitials, produce high compressive residual stresses. During annealing interstitial nitrogen diffuses to inner boundaries or to the surface. High residual stresses are decreased.

Low-friction, high-endurance, ion-beam-deposited Pb-Mo-S coatings

Abstract: Thin solid lubricating coatings of PbMoS were deposited on steel substrates via ion-beam deposition. Coating endurance and friction coefficients under dry air sliding conditions were monitored with ball-on-disk tests; additional tribological testing was performed using a ball-on-flat reciprocating test rig to investigate intermediate sliding distances (100–32000 cycles). Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD), scanning Auger microscopy and micro-Raman spectroscopy were used to examine the structure, composition and chemistry of the coatings. Worn surfaces were characterized by optical microscopy and micro-Raman spectroscopy. The average endurance (at 1.4 GPa stress) of ion-beam-deposited (IBD) PbMoS coatings (thickness, 160–830 nm) containing 4–26 at.% Pb was 160000 revolutions, more than twice that of MoS2 coatings obtained by ion-beam-assisted deposition. In addition, the IBD PbMoS coatings had friction coefficients between 0.005 and 0.02, similar to the MoS2 coatings obtained by ion-beam-assisted deposition. Friction coefficients were monitored as a function of the contact stress and found to obey the hertzian contact model; measured interfacial shear strengths (S0 ≈ 12 MPa) were similar to those observed for MoS2 coatings. Although XRD and micro-Raman spectroscopy indicated that the IBD PbMoS coatings were initially amorphous, micro-Raman spectroscopy showed that crystalline MoS2 was produced both in the wear tracks on coatings and in the transfer films on balls after as few as 100 sliding cycles. The wear resistance and low-friction properties of IBD PbMoS coatings are attributed to the combination of dense, adherent coatings and the formation of easily sheared, MoS2-containing sliding surfaces.

Wear resistant composite coatings deposited by electron enhanced closed field unbalanced magnetron sputtering

Abstract: There is presently considerable interest in wear resistant coatings produced using closed field unbalanced magnetron sputtering technology For example, layered films of diamond-like carbon (DLC) with tungsten or titanium additions have been widely reported The benefit is that the mechanical properties are enhanced (eg giving greater toughness); also it is possible to control the stress state and enhance adhesion Here we report the further development of this concept by the addition of TiN, TiCN and TiC layers in DLC-based composites, utilizing an additional source of electrons in the vicinity of substrate to enhance ionisation of the plasma and increase coating density Composite coatings of ceramics TiN, TiCxNy, TiC, CrN, TiCrN, TiCrCN, TiCrC, metal doped Tix%-DLC and their combinations were deposited on 316 stainless steel substrates The mass flow of reactive gases into the chamber was controlled using plasma optical emission monitoring to achieve the desired coating composition The morphology of the coatings was investigated and correlated with Knoop microhardness, scratch adhesion, pin-on-disc and wet abrasive wheel tests Dense T-type structures were found for most of the coatings and a high toughness of Ti30%-DLC coating with a TiC interlayer was observed Low friction coefficients of 015–018 for coatings with Tix%-DLC layers confirmed their benefit in sliding wear applications, while TiCN coatings were found to be the best in abrasive wear conditions

Hydrogen plasma chemical cleaning of metallic substrates and silicon wafers

Abstract: The plasma chemical cleaning process based on an argon-hydrogen discharge differs from conventional plasma cleaning methods. Chemical reactions are used for the removal of surface contamination and the sputtering of material is avoided. Therefore no problems due to the redeposition of the sputtered material occur. The process chemistry is confirmed by in situ measurements of the plasma during the cleaning procedure using a plasma monitor. The process was investigated for bare copper lead frames and for silicon wafers. Volatile hydrogen compounds were formed during cleaning. An Auger spectrometer was attached to the cleaning chamber to investigate the substrate surfaces after each cleaning step. It was shown that the carbon and oxygen contamination at the substrate surface could be reduced below 0.5 at.% (noise limit of the measurement). The results clearly show that this simple and environmentally friendly process is an effective method for reducing organic and some inorganic contamination.

On the synthesis of oil-containing microcapsules and their electrolytic codeposition

Abstract: Composite coatings consisting of a metal matrix in which ceramic particles are embedded have recently been developed and used in industry as wear-resistant coatings. The present paper deals with the development of oil-containing self-lubricating metallic coatings. These have been produced by electrolytic codeposition of oil-containing microcapsules from Watts nickel plating baths. For this purpose, oil-containing polyterephthalamide microcapsules were synthesized based on the interfacial polymerization of an oil-soluble monomer (terephthaloyl dichloride) and a mixture of two water-soluble monomers (diethylenetriamine and 1,6-hexamethylenediamine). The influence of several synthesis parameters (e.g. type of encapsulated organic phase, monomer concentration(s) and concentration ratio of the two amine monomers) on the size distribution and morphology of the oil-containing polyamide microcapsules as well as on their electrolytic codeposition behaviour is discussed. As revealed by scanning electron microscopy analysis, the morphological characteristics of the microcapsules were affected to a great extent by the functionality of the water-soluble amine monomer. Furthermore, the composition of the core material of the microcapsules showed a marked influence on their stability upon aging in the Watts nickel plating bath. Finally, codeposition experiments using a laboratory rotating electrode showed that the level of codeposition was influenced by the presence of additives in the nickel electrolyte and was strongly dependent on the polymerization conditions employed in the microcapsule synthesis.

Plasma-deposited passivation layers for moisture and water protection

Abstract: The barrier properties of electron cyclotron resonance plasma deposited silicon oxide (SiO), silicon oxynitride (SiON) and silicon nitride (SiN) films against moisture and water penetration were studied as a function of the deposition parameters gas flux ratios, microwave power and reactor pressure. Three methods were used to quantify these barrier properties: measurement of film etch rate in water at different temperatures and pH values, determination of moisture permeation coefficient and electrical characterization of samples during humidity exposure. Although the dissolution rate of SiN in water is slightly higher compared with SiO the lower moisture permeation coefficient and the higher electrical stability during exposure to humidity of silicon nitride make it an attractive passivation material for different applications.

Adhesion of advanced overlay coatings: mechanisms and quantitative assessment

Abstract: Functional properties and adhesion determine the performance of overlay coatings. The adhesive strength transmits the surface loads into the component and takes up the stresses that arise because of the differing properties of coating and substrate. Therefore, it is important to understand the mechanisms involved and to develop testing methods to measure the adhesive strength. Generally, three basic mechanisms of adhesion can be distinguished: (1) mechanical interlocking, (2) physical bonding, (3) chemical bonding. Depending on the coating/substrate system and deposition process, one mechanism usually plays a dominant role, although the others are present as well. In this way, a three-dimensional interfacial zone is formed, the width ranging from one atomic layer to several millimetres, again depending on materials, processes and parameters. Measurements of adhesive strength are often problematic, particularly with thin coatings and at high adhesive strength levels. For this reason numerous assessment tests have been developed, e.d. pull-off test, shear test, scratch test, indentation test, etc. An interesting solution to the problem is the fracture mechanics approach, where crack propagation in the interface is assessed. The paper discusses adhesion and adhesion testing with several examples of advanced coating systems.

Plasma surface treatment and PACVD on Ti alloys for surgical implants

Abstract: To improve the poor surface hardness and wear resistance of titanium alloys used for surgical implants undergoing wear TiA16V4 and TiA15Fe2.5 have been plasma nitrided. TiN and other sublayers are formed by plasma nitriding. To increase the thickness of the outermost TiN layer the samples have been coated with TiN by PACVD following plasma nitriding. The produced layers have been characterized in terms of microstructure, layer composition, hardness depth profiles. The evaluation of corrosion resistance and biocompatibility revealed that the good properties of the untreated alloys have been well preserved.

State of the art for the industrial use of ceramic PVD coatings

Abstract: Since the beginning of the 1980s ceramic PVD coatings have been used in industry for many applications. At present four types of PVD coatings are frequently used: TiN, Ti(C,N), (Ti,Al)N and CrN. Based on a materials triangle approach, it is shown which coating should be used for which application. Different industrial case studies will demonstrate the effectiveness of PVD coating. Some recent developments, of which various aspects are still mainly in the research phase, like diamond(like) coatings, duplex treatments, self lubricating coatings, Ti2N coatings and PVD coatings deposited at low temperatures (<250 °C), are also described.

Investigation into the impact wear behaviour of ceramic coatings

Abstract: Ceramic coatings with good impact wear resistance, hardness, toughness and adhesion are required in many applications. Coatings deposited by physical vapor deposition processes (CrN over an ENiP interlayer, TiC x N y , TiN and Ti x % -diamond-like carbon) have been evaluated using a specially designed cyclic impact tester. The test allows the modelling of wear under repetitive high local loads, as encountered by machine parts subjected to impact or tools used in interrupted cutting or cold forming. To adjust the test conditions for different applications, the impact force and frequency are variable and each impact can be monitored using a piezo-force transducer. A comparison of the results of the tests on the coatings studied shows differences in the characteristics of the impact wear in each case. Scanning electron microscopy and optical inspections of the wear regions allowed the creation of a model to describe hard coating degradation under repeated impact loading. Three different zones were identified: a central zone of cohesive failure; an intermediate zone of cohesive-adhesive failure; and a peripheral zone with circular cracks. Furthermore, it was found that the wear increased at the macrocracks, depending on the number of impacts, before coating delamination or spallation started to occur. The TiC x N y coatings and, in particular, the TiC 0.75 N 0.25 coatings were found to have better resistance to this kind of dynamic impact wear than was the case for the coatings with an ENiP interlayer. The T i x% -diamond-like carbon coatings showed good wear resistance in the cavity of the indentation area until a high number of impacts was reached, but coating delamination at the high shear stress zones (at the edges and centre of the cavity) took place after a lower number of impacts.

Laser alloying of aluminium alloys with chromium

Abstract: The microstructure and corrosion resistance of laser-alloyed aluminium and ANSI 7175 aluminium alloy with chromium were investigated. Surface layers alloyed with chromium contain relatively large amounts of intermetallic compounds dispersed in a matrix of α-Al. The intermetallic compound particles present needle-like morphologies, organized in a dense network or distributed radially. Al 7 Cr, Al 11 Cr 2 and α-Al phases have been identified by X-ray diffraction. The alloyed layers may contain cracks, pores, inclusions and undissolved chromium particles, depending on the chromium concentration and the particle size. However, homogeneous layers were produced by a two-step process, consisting of laser alloying followed by remelting. The second treatment eliminates porosity and refines the structure. The hardness attains a Vickers hardness of 155 HV in chromium-alloyed aluminium and exceeds 300 HV in chromium-alloyed 7175. The corrosion behaviour of the above alloys was assessed using anodic polarization techniques. Laser alloying of aluminium and 7175 with chromium improves the pitting corrosion resistance of the alloys. The effect depends on the chromium content of the alloyed layers and is more significant in 7175 alloy.

Vacuum arc coatings for tools: potential and application

Abstract: Cathodic vacuum arc evaporation is widely used to deposit hard wear-resistant coatings onto tools and machine parts. This paper reviews the potential for deposition of different hard coatings, e.g. MeN, (Me 1 , Me 2 )N, Me(N,C) in monolayer and multilayer forms as well as in the form of combination coatings. Specific properties of selected coatings are discussed in view of their applications for tools.

Application of diamond-coated cutting tools

Abstract: Today, diamond-coated cutting tools are used primarily for machining non-ferrous materials such as aluminium-silicon alloys, copper alloys, fibre-reinforced polymers, green ceramics and graphite. The tool life of cemented carbide cutting tools is greatly improved by diamond coating, and typically more than 10 times the tool life is obtained. In this report we will present cutting performances of diamond-coated inserts, twist drills, square end mills and ball nose end mills. Diamond films are usually deposited more than 10 pm thick to make tool life longer, since tool life is directly related to film thickness. However, increased film thickness caused many problems with cutting performance. The most severe problem was the decrease in the transverse rupture strength of the diamond-coated substrates. Because of this effect, the diamond-coated insert used under high speed and intermittent cutting conditions was supposed to suffer chipping easily.

Characterization of low temperature CrN and TiN (PVD) hard coatings

Abstract: The plasma beam sputtering process was used to prepare CrN and TiN hard coatings on steel, sapphire and alumina Superstrate substrates at a temperature of 200 °C. The microstructural characteristics, coating morphology, interfacial properties, microhardness and internal stresses were studied for coatings 3 μm thick. Their oxidation behaviour in an oxygen flow in a tube furnace was studied at temperatures up to 600 °C for TiN and up to 800 °C for CrN coatings. The initial stage of oxidation of TiN and CrN coatings 350 nm thick was also studied by continuous in-situ electrical resistivity measurements. This measurement technique offers high reproducibility and accuracy, so can be used to study oxidation for all types of hard coating, and especially for new multilayer and duplex coatings. Weight gain measurements and Auger electron spectroscopy depth profile analyses of oxidized stoichiometric CrN coatings showed that oxidation in an oxygen flow at 800 °C for 4 h induced the growth of a stable Cr2O3 film. The thickness of this oxide film was only 15% of the total (3 μm) coating thickness. The surface morphology and small roughness changes — as also observed after a long-term test at 800 °C for 200 h in oxygen — indicated that a stoichiometric CrN coating with the measured properties can be successfully used as a hard and oxidation-resistant coating in industrial practice, even if the working temperatures are higher than 750 °C.