E. Broszeit

Corrosion performance of some titanium-based hard coatings

Abstract Tools and machine parts which could benefit from wear-resistant titanium-based hard films are often subject to corrosive environments. Physically vapour-deposited coatings frequently exhibit porosity and even small defects, which can cause rapid local corrosion of the substrate material; there is therefore a requirement for dense and chemically inert coatings. This paper presents corrosion data for titanium-based hard coatings such as TiN, (Ti,Al)N, Ti(B, N) and TiB2 and also for multilayered structures where additional aluminium-based insulating surface layers (AlN and Al2O3) were deposited. The corrosion resistance and porosity of the films were analysed by electrochemical techniques. The degree of metallic bonding can play a significant role in influencing the corrosion resistance of refractory transition-metal-based ceramic coatings. Here we demonstrate that, under potentiodynamic corrosion test conditions, resistance to corrosive attack was relatively poor for TiB2, better for (Ti, Al)N and Ti(B, N) and best for TiN. It is also shown that applying the additional protective aluminium-based insulating surface layers on the coating can further improve corrosion resistance.

Fundamental properties and wear resistance of r.f.-sputtered TiB2 and Ti(B,N) coatings

Thin films of TiB2 and Ti(B,N) were deposited by means of r.f. sputtering. Fundamental properties such as structure, adhesion, hardness and internal stresses were determined as a function of the deposition parameters. By systematically changing the substrate voltage and in the case of Ti(B,N) the nitrogen flow, the film properties could be influenced over a wide range. The coatings have been tested with regard to their tribological behaviour by means of a model wear-testing apparatus. It is shown that Ti(B,N) is a promising coating for wear protection.

A comparative study of the corrosion performance of TiN, Ti(B,N) and (Ti,Al)N coatings produced by physical vapour deposition methods

Abstract Thin film coatings produced by physical vapour deposition methods often exhibit porosity. Local defects can cause local and rapid corrosion of the base material. The porosity is difficult to estimate and electrochemical methods are most suitable for evaluating the corrosion resistance of the coated material. This paper compares the corrosion resistance of TiN, Ti(B,N), (Ti,Al)N- and TiB 2 -coated ASP 23 high speed steel. For the materials studied here the corrosion performance of TiB 2 -coated samples was poor. Ti(B,N) coatings obtained by two different methods were quite similar even though the calculated porosity of the coating produced by magnetron sputtering was lower than that of coatings produced by the electron beam technique. These coatings had similar or slightly better corrosion resistance than (Ti,Al)N coatings with a high aluminium-to-titanium ratio. (Ti,Al)N coatings with a low aluminium-to-titanium ratio were better than coatings with a high aluminium-to-titanium ratio. TiN coatings were better than other types excluding (Ti,Al)N + AlN layer coatings, which performed best. (Ti,Al)N + AlN coatings have an insulating layer on top of the coating, which increases the polarization resistance and decreases the corrosion current density.

Structure, internal stresses, adhesion and wear resistance of sputtered alumina coatings

Abstract Thin alumina coatings have been produced by means of reactive r.f. magnetron sputtering. The layers, deposited onto metallic substrates, show an amorphous crystallographic structure. Scanning electron microscopy analysis shows that the structure is variable within a wide range, from rough fibrous to smooth glass like, depending on the deposition parameters. Measurement of internal stresses was performed by using the bending beam method. The magnitude of the measured compressive stresses depends on the film thickness, deposition pressure and sputtering power. Some correlations between internal stress and adhesion of the coatings are evident. As well as scratch-test measurements the amount of layer detachment of bending specimens gave information about the adhesion behaviour of the coatings. Some of the alumina coatings have been tested with regard to their tribological behaviour by means of a model testing apparatus. The coatings showed excellent wear behaviour even under severe tribological conditions.

Tribological properties of r.f. sputtered Ti-B-N coatings under various pin-on-disc wear test conditions☆

Abstract TiN, Ti(B,N) and TiB 2 hard coatings have been deposited on steel substrates by means of r.f. sputtering. The fundamental coating properties, such as the structure, adhesion, hardness, internal stresses and chemical composition, have been determined. The coatings have been tested with regard to their tribological behaviour by means of unlubricated pin-on-disc model wear tests under an ambient atmosphere as well as under vacuum conditions. Balls made of ball bearing steel and high speed steel have been used as pin materials. It was found that the dominant type of chemical bonding within the film is an important coating property with regard to the wear behaviour under pin-on-disc test conditions. It is shown that TiB 2 films are of limited benefit in a sliding contact against steel. Ti(B,N) and the well-known TiN are much more promising coating materials.

Influence of substrate material and deposition parameters on the structure, residual stresses, hardness and adhesion of sputtered CrxNy hard coatings

Hard coatings of CrxNy were sputtered on various substrate materials such as steel SAE 52100 and titanium alloy Ti6A14V which are commonly used in the aerospace industry. The coatings were investigated for their fundamental and mechanical properties. It was found that there are good correlations between structure, hardness, residual stresses, cohesion and adhesion of the coatings.

Tribological behaviour and corrosion performance of Ti-B-N hard coatings under plastic manufacturing conditions☆

Abstract During plastics processing, an increasing content of abrasive fillers and corrosive components in combination with high working temperatures up to 450°C produce extreme wear conditions. As shown in previous work, TiN coatings are wear resistant against hot molten plastic material. In this paper we deal with thin hard single-phase and multiphase coatings within the extended coating system Ti-B-N. The films were deposited onto steel substrates using an r.f. sputter process with r.f. substrate bian. In order to cover a broad spectrum of Ti-B-N coatings, the substrate bias, nitrogen gas flow and total deposition gas pressure were all varied systematically. The coatings were tested with regard to their fundamental mechanical properties such as hardness and adhesion. Additionally, the coating structure was investigated by scanning electron microscopy. Corrosion tests were also done to see how the coatings performed under this important condition. Following these tests the wear behaviour of the coating systems under plastics processing conditions was investigated in a model wear test. Corresponding to the melting zone of screw barrel systems, the test pieces were subjected to abrasive-corrosive wear under elevated temperatures. It is shown that the combination of high hardness, dense structure, good adhesion and—most important—excellent corrosion resistance leads to superior wear resistance of r.f. bias sputtered Ti-B-N coatings. The wear behaviour can be further improved if a dense covalent Al2O3 surface layer is deposited on top of the Ti-B-N film.

Analysis of r.f.-sputtered TiB2 hard coatings by means of X-ray diffractometry and Auger electron spectroscopy

Abstract The bias voltage can be a very important parameter in r.f. sputtering. Auger electron spectroscopy (AES) and X-ray diffraction (XRD) were used to investigate the influence of this parameter on TiB 2 hard coatings. As a result of the AES measurements the boron-to-titanium ratio depends on the bias voltage. With XRD, significant differences in the various coatings can be observed.

PVD hard coatings on prenitrided low alloy steel

Abstract The combination of traditional surface treatments such as nitriding with modern plasma-enhanced surface technologies reveals the possibility, particularly in the application to low alloy steels, of obtaining mechanical properties comparable with those of high alloy steels. Gas-nitrided samples of the hardened and tempered low alloy steels 30CrMoV9 and 17CrMoV10 were TiN coated by r.f. magnetron sputtering and ion plating. The requirements to obtain a nitrided substrate that can be coated were given special consideration. For this, various surface modifications of the nitrided substrates were realized by bright nitriding, nitriding with a compound layer and additional steps before coating, such as polishing, grinding and sputter cleaning. The properties of prenitrided coated steels essentially depend on the structure and properties of the outer part of the nitrided case. TiN on bright nitrided and nitrided substrates with the compound layer removed has a better adherence than on compound layers. The decomposition of the iron nitride during the plasma sputter cleaning of compound layers results in a lower surface hardness and lower adherence of TiN. The highest wear resistances in the Timken test were registered on samples where the compound layer had been removed before TiN coating.

Comparative tribological and adhesion studies of some titanium-based ceramic coatings

Abstract In this work different evaluation methods for thin hard coatings were studied in three laboratories. Different coatings and test equipment were used. An agreement on the procedure for testing and data appraisal was made. The results obtained in the different laboratories varied depending on the test procedure used. Two test methods, ball crater and scratch test, yielded smaller variations in the results, whereas the hardness measurements and pin-on-disc tests had larger variations, although in the former case these were within the standard deviations of the readings. It is clear from this work that tests performed in different laboratories cannot be used for comparison purposes, unless strict agreement is reached on the exact test procedure and on the basis of interpretation of the results. Even tests carried out under nominally identical conditions in one laboratory can in some cases give a spread in performance, since many variables, such as friction coefficient, wear rate and hardness, are not intrinsic materials properties and can statistically vary. It is thus important to devise standardized test methods, and in all cases to quote the standard deviations on data.