Peter Panjan

PVD coatings as an environmentally clean alternative to electroplating and electroless processes

Abstract Today the development of clean technologies in all spheres of industrial manufacturing is an essential task, not only for material and metal finishing but also for plasma surface engineering. Among the most critical group of technologies which needs to be replaced by alternative technologies are processes used to produce functional galvanic and decorative coatings. The electroplating of finishes, such as hard chromium, cadmium and nickel in metal finishing is today recognized as a major source of environmental pollution in every country. Therefore wet bath technologies have started to lose favour compared with high performance dry coating methods such as physical vapour deposition (PVD), plasma-assisted chemical vapour deposition, chemical vapour deposition and thermal spraying. Among these techniques, the results obtained with PVD coatings in metal cutting and forming in the last 15 years show the most promising solution of the complicated situation in which galvanic coatings seemed to be technologically and economically irreplaceable. In this paper the general situation in this field is shown. Already today it is possible to replace efficiently some of the galvanic processes in specific cases (e.g. Cr, Ni, Cd, Zn, Au). It is important to point out that PVD is considered to be a technique which can provide not only metallic, but also alloyed and ceramic coatings with a virtually unlimited range of chemical composition and therefore controlled protective, mechanical and wear-resistant properties. Entering into competition with galvanic coatings the manufacturers of PVD coaters were confronted with new requirements: a huge quantity of substrates of the same size, to be chemically and plasma cleaned and then coated at the highest possible deposition rate. For industrial mass production one can therefore use only large PVD batch systems or in-line coaters. The alternative for todays low price galvanic coatings is therefore dry and clean PVD technologies, fully supported by legislation on environmental protection. The economics depend directly on the substrate type and the quantity. The first positive results on the replacement of electrodeposited nickel on aluminium substrates and hard chrome on soft iron are also reported here. A soldering test was made on a sputtered nickel layer. Wear and corrosion tests were performed with iron cores, coated with PVD CrN coating. All tests were made in the Slovenian automotive industry. Results show that for a large number of substrates PVD clean technology is already economically competitive with galvanic coatings.

Oxidation of TiN, ZrN, TiZrN, CrN, TiCrN and TiN/CrN multilayer hard coatings reactively sputtered at low temperature

Abstract All the coatings and multilayer structures were deposited in a Sputron plasma-beam-sputtering apparatus at a temperature below 150 °C on polished alumina substrates (Ra= 25 nm) and polished tool steel discs. The coatings were annealed in an oxygen flow at temperatures in the range 500–850 °C. The oxidation kinetics of these coatings were studied by means of the weight gain as a function of the oxidation time at each temperature. On the same coatings, continuous in situ electrical resistivity measurements were taken and these indicate grain boundary oxidation. The composition and the depth profiles of the oxidation products formed were investigated using AES and XPS combined with sputter profiling. The oxide layers were found to grow according to a parabolic diffusion law with activation energies of 1.04, 1.89, 1.92, 2.37,2.91 and 3.98 eV for Ti0.63Zr0.37N, TiN, CrN, ZrN, TiN/CrN and Cr0.72Ti0.28N, respectively.

Comparison of structural and chemical properties of Cr-based hard coatings

Abstract A series of chromium nitride, carbide and carbonitride coatings were prepared by ion plating using nitrogen and acetylene as reactive gases. The depositions were made at different partial pressures of the reactive gases while maintaining other parameters constant. The crystal structure and microstructure were studied by TEM. The CrN and Cr 2 N phases were detected in the chromium nitride coatings. In the carbide coatings, orthorhombic Cr 7 C 3 and the metastable cubic CrC phase were confirmed. For oxidation studies, the samples were annealed at 750 °C. Afterwards, the depth profile was measured by glow discharge optical emission spectrometry (GDOES). A complex diffusion was observed involving outdiffusion of nitrogen and segregation of carbon at the substrate-coating interface.

Improvement of hot-working processes with PVD coatings and duplex treatment

Abstract Optimisation of manufacturing processes in mechanical engineering which involve working temperatures higher than 600°C and high surface loads, e.g. pressure die-casting, hot extrusion and hot forging, is today under intensive investigation worldwide. Traditional production procedures and the enormous quantities of products in these industries are serious limitations to the introduction of any kind of improvement to moulds, tools and dies with PVD coatings and duplex treatment. Thus, the simulation of selected tribo-systems is practically impossible and any performance testing of new technologies must therefore be carried out in real industrial manufacturing. In this paper we discuss the properties of surface improvements relevant to these topics. These comprise PVD CrN, TiAlN and FUTURA TiN/TiAlN multilayer coatings, and duplex treatments, including plasma nitriding (PN) and all three PVD coatings. We studied the applications of CrN, PN+CrN and PN+TiAlN in aluminium pressure die-casting, CrN in hot extrusion of Al and FUTURA and PN+FUTURA coating in hot forging of steel parts. Performance tests were carried out in various industrial plants in Slovenia. The results show that an improvement in tool or die life (cost saving) of up to several 100% is not the decisive factor. More important for these traditional industries are better reproducibility of the surface quality of the products, an increase in the manufacturing reliability in heavy machining, a decrease in manufacturing interruption (down-time) and finally, the protection of the environment.

A new experimental method for studying the cracking behaviour of PVD multilayer coatings

Abstract One way to improve the cracking resistance of PVD hard coatings is to prepare them in the form of a multilayer structure, where micro-cracks tend to branch and deflect at the interface between two alternating layers of different materials. However, it is still not clear how a layer structure influences the cracking resistance. One of the main reasons for the shortage of data in this field is the lack of appropriate experimental methods to prepare the samples and to observe the micro-cracks that appear through the multilayer structure. Due to the extremely small layer thicknesses, a SEM analysis on a perpendicular sample cross-section is too coarse, while a cross-sectional TEM analysis is time consuming and, therefore, expensive. In this study a new experimental method for observating the propagation of micro-cracks in different multilayer structures is presented. It is based on microscopic observations of the micro-cracks on the cross-section of a ball crater over a scratch track. The first step of this test is to prepare a scratch on the sample using a diamond stylus. Next, a ball crater must be ground over the scratch track at the position where the first cracks are observed in the scratch track. Along the edges of the scratch track the low-angle cross-section of the multilayer structure can be observed. On such a cross-section we could clearly see how the micro-cracks propagate from the coating surface through all the layers that compose the multilayer structure. For the inspection of the micro-cracks we used optical as well as SEM microscopy. The applicability of this new technique was tested on a 2.6 μm-thick TiN/CrN multilayer structure with 6, 8, 16 and 32 layers and on a 2.4 μm-thick CrN/TiAlN multilayer structure with 8 layers. The coatings were prepared by reactive sputtering in a Sputron (Balzers) plasma–beam sputtering apparatus. The coating adhesion was characterized with a conventional scratch tester (CSEM Revetest). For the ball–crater preparation we used a steel ball with a diameter of 20 mm and a diamond paste with a grain size 0.25 μm. The ball craters were prepared on the scratch track at the positions that corresponded to the different scratching loads. We discovered that the individual layer structure has a strong influence on the crack propagation as well as on the plastic deformation of multilayer coatings.

PVD CrN coating for protection of extrusion dies

In this paper, we present a comparison of tribological behaviour between conventional gas nitrided and PVD CrN coated extrusion dies used in industrial production. We found that PVD CrN coated dies proved to be superior to nitrided ones.

Improvement of hot forging manufacturing with PVD and DUPLEX coatings

Abstract The application of a DUPLEX treatment, which included low pressure pulsed plasma nitriding of AISI H11 tool steel dies and PVD BALINIT® FUTURA-TiN/TiAlN multilayered coating, was studied as a possible technology to improve the process of hot forging of steel components. The quality of the DUPLEX treatment was determined by laboratory analyses of coated test plates, which were treated together with the hot forging dies. Surface roughness, Vickers microhardness including in-depth measurements, coating thickness and adhesion parameters determined by the REVETEST method and microanalysis (SEM and EDS), were investigated. Performance tests were made in real working conditions in the UNIOR hot forging industry in Zrece, Slovenia, and included three types of technological processes: (a) form milling of graphite electrodes (to produce dies by the electrical discharge machining process with cemented carbide cutters improved with a BALINIT® Diamond coating); (b) hot forging of steel components (with two step forging dies fixed in a 1650 t eccentrically driven press, improved with the DUPLEX treatment); (c) hole punching in forged parts (with dies made of ASP30 tool steel and improved with a standard PVD TiN coating). The hole punching in forged parts and milling of graphite electrodes were performed successfully in practice for many years. In hot forging, the first positive results were obtained after three experimental studies in the last two years. Optimisation of the parameters of the DUPLEX treatment is still needed before this technology can be introduced into regular manufacturing operations for hot forged steel parts.

High-temperature oxidation of TiN/CrN multilayers reactively sputtered at low temperatures

The high-temperature (600–850°C) oxidation of multilayer hard TiN/CrN coatings was investigated. TiN/CrN multilayer coatings (2.6 μm thick) with modulation periods in the range 83–425 nm were deposited at low temperatures (150°C) onto polished tool steel discs and polished alumina ceramic substrates (Ra = 25 nm) using the plasma-beam sputtering technique. The oxidation rate was obtained from the weight gain as a function of the oxidation time. The activation energy for oxidation of TiN/CrN multilayers was determined and compared with that for CrN, TiN and CrTiN coatings. The depth profiles of samples oxidized at various temperatures were examined by Auger electron spectrometry (AES) and glow discharge optical spectrometry (GDOS) to determine the thickness and the composition of the oxide layer. The morphology of the oxidized coatings was investigated by scanning electron microscopy (SEM). Continuous in-situ electrical resistivity measurements were performed on the same coatings to detect grain-boundary oxidation. The adhesion and microhardness of the multilayers were also determined.

Properties of Cr(C,N) hard coatings deposited in ArC2H2N2 plasma

Abstract Several chromium carbonitride (Cr(C,N)) coatings were prepared with different C:N ratios by varying the N 2 and C 2 H 2 flow. Chromium nitride (CrN) and chromium carbide (CrC) coatings were also prepared for comparison. The coatings were deposited in two different ion-plating systems: by reactive evaporation in BAI730M (Balzers) apparatus at high temperature (450 °C) and by reactive sputtering in plasma-beam Sputron (Balzers) apparatus at low temperature (200 °C). Among mechanical properties microhardness, adhesion (measured by scratch test) and surface roughness were evaluated. Oxidation of the coatings was carried out by heating the samples at temperatures of 750–900 °C in an oxygen atmosphere. Crystal structure and microstructure were studied by XRD, TEM and SEM. Chemical State of the elements was observed by XPS. The concentration and depth profiles of the samples oxidized at various temperatures were measured by AES, EDX and GDOES.

Energy resolved ion mass spectroscopy of the plasma during reactive magnetron sputtering

Abstract The energy distribution and composition of the ion flux on a substrate during reactive magnetron sputtering of TiN and TiWN films were studied by the energy resolved mass spectroscopy. The entrance flange of the probe Hiden EQP500 was positioned in a distance of 50 mm from the Ti or WTi (70:30 at.%) target 100 mm in diameter. The sputtering was carried out in a mixture of argon and nitrogen of various compositions at pressures from 0.05 to 10 Pa and discharge currents from 0.5 to 7 A. The energy spectra of ions at low pressures were characterized by extended high-energy tails. The high energy of sputtered (metal) atoms follows from their distribution at the cathode after being sputtered. The high-energy gas ions (Ar + , N 2 + , N + ) stem from two sources. One is the transfer of energy in the collisions with the sputtered metal atoms. The other is the reflection of the energetic ions from heavy elements in the target. A strong reduction of the ion energy at the substrate was found when the pressure was increased from 0.5 to 10 Pa. As a consequence of a loss of energy in many collisions the high-energy portion of the ion energy spectra diminished and the energy spectra of various kinds of ions became similar. Nevertheless, the reflected ions were still apparent, albeit at lower intensity. The TRIM Monte-Carlo simulation showed that the flux of the fast reflected ions and flux of sputtered atoms are of the same order of magnitude, indicating thus the important role of the former species in forming the film properties at low pressures. The analysis of the composition of the ion flux during sputtering in a mixture of nitrogen and argon revealed that the ratio of ion fluxes TiN + /Ti + reached maximum of approximately 0.13, while WN + /W + was up to 0.3.