E. Harju

Comparison of cutting performance of ion-plated NbN, ZrN, TiN and (Ti, Al)N coatings

Abstract Three binary nitrides of the refractory materials titanium, zirconium and niobium and ternary (Ti, Al)N as well as quaternary (Ti, Al, V)N coatings were deposited by reactive triode ion plating on powder metallurgically produced high-speed steel (PM-HSS) inserts. The coated tools were then tested by dry turning of hardened and tempered AISI 4140 steel. The effect of the deposition parameters on coating properties was studied in detail. The results showed that NbN coating does not seem to improve the wear resistance in the turning of steel. Titanium-based nitride films such as TiN and (Ti, Al)N, however, appeared very suitable, as they showed increasing cutting performance and finer structures with increasing substrate current density. ZrN, however, showed the opposite effect. The best turning properties were achieved with a substrate bias of −100 V. Increasing thickness improved cutting performance almost linearly in the range from 1 to 5 μm. Substrate polishing before the deposition of the TiN layer had a strong effect, almost corresponding to doubling of the coating thickness. For example, a thickness increase from 3 to 5 μm yielded a performance improvement which could also be achieved simply by polishing the substrates carefully before coating. The three best coating materials in decreasing order of performance were ZrN, (Ti, Al)N and TiN. The differences between the best coating materials were generally not large.

Comparison of the corrosion resistance of TiN and (Ti,Al)N coatings

Abstract The corrosion resistance of various commercially available TiN-based physically vapor deposited coatings were compared. All coatings were deposited onto stainless steel substrates. The corrosion resistance of the samples was studied using anodic polarization and potentiostatic measurements. The film structure and substrate-coating interface chemistry were studied using X-ray diffraction, scanning electron microscopy (SEM) and secondary ion mass spectroscopy (SIMS). All samples had good corrosion resistance under short-term anodic polarization measurements but clear differences could be observed in the long-term potentiostatic experiments. The corrosion resistance was found to be based on two factors: (i) the degree of porosity in the thin film and (ii) the nature of the coating-substrate interface. The proper chemistry of the coating-substrate interface appeared to be important in achieving good corrosion resistance. TiN samples with proper chromium and chromium oxide distribution at the substrate-coating interface together with one (Ti,Al)N coating chosen for the study showed the best corrosion performance.

The influence of titanium interlayers on the adhesion of PVD TiN coatings on oxidized stainless steel substrates

Abstract It has been shown that the use of thin titanium interlayers improves the coating-substrate adhesion of physical vapour deposition (PVD) titanium nitride thin films on a stainless steel substrate. This improvement arises from a combination of chemical gettering and mechanical compliance effects. The improved adhesion of plasma-assisted chemical vapour deposition TiN coatings with increasing interlayer thickness has been shown to be largely attributable to the compliance effect (S.J. Bull, P.R. Chalker, C.F. Ayres and D.S. Rickersby, Mater. Sci. Eng. A, 139 (1991) 71). The development of practical methods to improve adhesion is hampered by the difficulties involved in quantitative measurements of the effect. To avoid the influence of the intrinsic and extrinsic parameters involved in scratch test and microhardness measurements, efforts have been made to apply fracture mechanical testing methods to the determination of the adhesion strength of the film on the substrate (S. Berg, S.W. Kim, V. Grajewski and E. Fromm, Mater. Sci. Eng. A, 139 (1991) 345). In our study the influence of Ti interlayers on the adhesion of PVD TiN coatings on oxidized stainless steel substrates was investigated using a pull-off test for adhesion measurements and scanning tunnelling microscopy and secondary ion mass spectrometry for analysis of the fractured surfaces. It was shown that the thickness of the Ti layer must be chosen according to the thickness of the oxide layer. An excess of Ti leads to lower adhesion values due to failure in the Ti layer, while a shortage of Ti leads to unreacted oxide and minimum adhesion due to brittle fracture in the oxide layer, which was shown to be amorphous.

Formability and corrosion resistance of TiN-coated stainless steel sheet

Abstract Refractory binary nitrides such as TiN used as coatings can improve the corrosion resistance of stainless steels. However, the necessary prerequisite is that the coatings are defect-free. Recently it has been proposed that TiN should be deposited directly on stainless steel strip and that moderate plastic forming of the TiN-coated steel may be possible without appreciable cracking. Results from uniaxial and biaxial tensile tests with TiN-coated stainless steels are reported. The surface integrity and possible formation of cracks were studied both visually and by using scanning electron microscopy and scanning tunnelling microscopy. Corrosion tests were carried out to evaluate the performance of the samples after forming. The tests showed that although TiN is a brittle material its adhesion to the substrate is surprisingly good. The coating thickness affected the cracking behaviour. The number of cracks decreased with increasing thickness. The crack pattern changed from transverse to oblique cracking at a particular strain level. The corrosion resistance of the TiN-coated steel was decreased by subsequent forming but it still exceeded that of the stainless steel substrate.

Optimization of wear and corrosion resistance of triode-ion-plated nitride coatings

Abstract Binary nitrides of titanium, zirconium and hafnium were deposited by triode ion plating onto high speed steel (HSS) inserts and HSS rods. The TiN and ZrN coatings consisted of stoichiometric coatings and coatings comprising alternate nitrogen-rich and nitrogen-poor layers. The nitrogen flow was varied during deposition to produce coatings with different structures. The inserts were tested by dry turning of quenched and tempered steel, and the results of cutting tests were compared with those obtained from the corrosion tests. The turning tests showed that cutting speed could be almost doubled compared with that for an uncoated insert. The stoichiometric TiN and ZrN coatings showed the best wear resistance, while coatings containing intermediate metallic layers were much poorer. The best combination of wear and corrosion resistance was achieved with a TiN coating comprising two stoichiometric layers and an intermediate Ti-N layer of nitrogen concentration about 35 at %.

Hard transition metal nitride films deposited by triode ion plating

Abstract The principles of reactive triode ion plating will be briefly described and examples of new nitride coatings deposited on cutting tools will be given. The coatings discussed include TiN, (Ti, Al)N and ZrN. Their relative order of performance in turning tests will be discussed and microstructural details will be studied by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It will be shown that, although the best coatings may appear rather uniform and hardly any structural details can be distinguished by SEM, cross-sectional TEM studies always reveal that a very-fine-grained columnar morphology exists in the coatings.

Surface engineering with light alloys—Hard coatings, thin films, and plasma nitriding

Light alloys have been attracting increasing attention over the past decade, since they can be used to reduce weight and save energy. For many years, light metals such as titanium and aluminum have also been used to synthesize hard compound coatings such as physically vapor deposited (PVD) TiN, (Ti,Al)N, and chemically vapor deposited (CVD) Al2O3. The coatings field is developing rapidly. Combining plasma-aided coating and diffusion processes has led to the development of so-called “duplex treatment,” consisting of plasma nitriding and subsequent hard coating. Another interesting development is TiN coating of aluminum vacuum parts, such as pumps, to reduce degassing and make the cleaning of the surfaces easier. Despite the many advantageous properties of light alloys, their surface properties sometimes cause problems. For example, galling may be a severe problem with titanium parts, and plasma nitriding has been applied successfully to combat it. However, due to adherent oxide scale, plasma nitriding of aluminum has proven to be more difficult. In this paper, we discuss some recent trends in the application of plasma-aided coating, thin film deposition, and diffusion processes, and give practical examples of industrial applications.

Effect of work material composition on the wear life of TiN-coated tools

Abstract Three commercially available quenched and tempered steels corresponding approximately to AISI 4140 were compared in dry turning using both uncoated and TiN-coated high-speed steel (HSS) inserts. Of three steels A, B and C, steel A did not contain added calcium, while both B and C were calcium-treated. In dry turning with uncoated HSS inserts steel B was best. It gave over 2 times longer wear life than steel A and 1½ times longer than steel C. When the inserts were coated with TiN, the cutting speed could be increased and the order of performance changed dramatically. Steel C was then best, giving nearly 26 times longer wear life than steel A and 9 times longer wear life than steel B. Based on secondary ion mass spectroscopy measurements, the enrichment of alloying elements was studied on the tool surface. The formation of an adherent protective layer on the rake face during turning of steel C is proposed as a mechanism explaining the observed differences in wear lives.

Formation of a wear resistant non-metallic protective layer on PVD-coated cutting and forming tools

Abstract Our previous studies have confirmed that the machinability of a Ca-treated steel is considerably improved compared to the untreated one, when the cutting is performed with a TiN-coated tool. When turning with TiN-coated HSS-tools the life time may increase by several hundred per cent, whereas with uncoated HSS-tools the life time increases only by tenths of a per cent. In this work both modified and unmodified commercially available quenched and tempered steels corresponding approximately to AISI 4140 were compared both in dry turning and forming at elevated temperatures. TiN-coated high-speed steel (HSS) and H 13 tool steel were used as tool materials. The inclusions in the steels and the existence and nature of the protective layer were studied using X-ray energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) as analytical tools. In XPS analyses, the enrichment of alloying elements was observed on the surfaces of cutting tools. Clear evidence of the formation of a non-metallic protective layer on the tool surface was found when turning Ca-modified steel with a TiN-coated tool. The results of the cutting tests showed that Ca cannot alone form the protective layer, but a combination of Ca and other steel alloying elements are needed to improve the machinability of the steel.

Structure investigations of wear reducing non-metallic layers on TiN-coated HSS turning tools

Abstract There has been considerable interest in improving the machinability of steels. The favourable effects of Ca to machinability were already found by Konig and Opitz et al. in the late 1950s. The modified, Ca-treated, quenched and tempered steels are now commercially available, but there seem to be considerable differences in performances between different producers. In our earlier study, both the modified steels performed better than the untreated steels, but the differences in machinabilities between the treated steels were considerable. The formation of the non-metallic protective layer was examined by SIMS and the inclusions in steels by EDS in order to find reasons for different machinabilities. In this study, the structural characteristics of the formed protective layers were investigated using X-ray photoelectron spectroscopy (XPS), cross-sectional transmission electron microscopy (X-TEM), X-ray energy dispersive spectroscopy (EDS) and atomic force microscopy (AFM). The X-TEM images proved the existence of the non-metallic layer on the surface of the tool after turning of both steels. The protective layers showed an uneven thickness and inhomogeneity in morphology and composition. The XPS and EDS analyses revealed the enrichment of the same elements, Ca, Mn, S and Al, on the surface of the layer, though the marks of these elements were more pronounced after turning of the better performing steel. The reason for the notable differences in machinabilities may therefore lie in the adhesiveness and uniformity of the layer on the surface of the tool.