C. Rebholz

Multilayer composite ceramicmetal-DLC coatings for sliding wear applications

Abstract The design of anti-friction coatings able to perform well in different wear conditions without lubricants requires a combination of adequate hardness and toughness, good adhesion, a low friction coefficient and a low wear rate. Recently introduced metaldiamond like carbon (DLC) coatings produced by magnetron sputtering of metals from targets, which are to a controlled extent covered with carbon from the chamber atmosphere, can be a step towards the achievement of such a combination. These coatings consist of an amorphous a:CH matrix with the possible incorporation of metal (Ta, W, Nb, Ti), metal carbide and/or graphite grains. Previous studies of Ti x% -DLC coatings showed their good protective properties against abrasive, impact and single scratch wear, as well as a requirement for supporting interlayers to successfully apply such coatings to low-cost steels. In the present work an example of the selection of metal-ceramic Ti-TiN-TiCN supporting interlayers is given based on studies of their morphology, structure and mechanical properties. This resulted in the development of Ti-TiN-TiCN-[TiC-(Ti x% -DLC)] multilayer composite coatings. Several coatings were prepared with the same supporting interlayer and a variation in the preparation of the Ti x% -DLC layer. Ball-on-disc experiments were carried out to investigate these coatings in conditions of sliding wear against steel and cemented tungsten carbide balls. CrN, TiN and TiCN coatings were also deposited and tested in the same conditions to provide a reference. Low friction coefficients (below 0.2 at an air humidity of 50% RH) in combination with low normalized wear rates were found for multilayer coatings with upper Ti 20% -DLC and Ti 35% -DLC layers.

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

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 (e.g. 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 0.15–0.18 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.

Structure, hardness and mechanical properties of magnetron- sputtered titanium-aluminium boride films

TiAlB films nominally 2 mm thick were produced by simultaneous sputtering from TiAl and TiB 2 targets onto Si-(110) and AISI316 stainless steel substrate materials at a temperature of 170°C. The influence of diVerent B/Al ratios on the structure, hardness and mechanical properties of coatings containing 32 at.% Ti was investigated, over a range of chosen compositions between B/Al=1 and B/Al=10. All coatings showed a very dense microstructure, and evidence of Al, Al 3 Ti, Ti and TiB 2 phasebonding was observed in X-ray diVraction and X-ray photoelectron spectroscopy analyses of selected coatings. Correlation of compositional and mechanical data showed that film hardness approximately trebled (from 12 to 35 GPa) and elastic modulus approximately doubled (from 185 to 340 GPa) with increasing B/Al ratio over the selected range. Reciprocating-sliding wear tests with both SAE52100 and WC‐6% Co counterface materials indicated that the lowest coating wear occurred at B/Al ratios greater than 2, although the wear rate increased slightly at B/Al ratios greater than 4, indicating a gradual reduction in coating toughness as the boron content exceeded 50%. All coatings with B/Alµ2 exhibited lower wear than untreated AISI316 substrates and, particularly in the case of the WC‐6% Co counterface material (for which the contact forces were higher), they were significantly better than commercial TiN of a similar thickness. These results demonstrate the advantages which relatively low modulus hard coatings can provide — particularly on softer substrate materials which provide limited load support.

Structure, mechanical and tribological properties of sputtered TiAlBN thin films

TiAlBN films were produced by simultaneously sputtering from TiAl and TiB 2 targets in Ar/N 2 mixtures at a substrate temperature of 150 °C. The goal of this investigation was to study the influence of the nitrogen content (0‐40 at.%) on two sets of four coatings with B/Al ratios of approximately 1 and 5. All coatings showed very dense structures and were mainly found to be only partially crystalline. Evidence of Al, AlN, Ti, TiB 2 , TiN and BN bonding was observed in the XPS spectra, and correlation of the compositional and mechanical data showed that film hardness and elastic modulus decreased with the presence of a ‘‘soft’’ phase (Al, Ti or amorphous BN ). Hardness and elastic modulus values of up to 25 and 250 GPa, respectively, were found for TiAl 0.3 B 1.7 coatings. Coatings with a similar elastic modulus to that of the steel substrate showed the best adhesion. Friction coeYcient values between approximately 0.5 and 0.7 were recorded in sliding wear experiments against SAE52100 and WC/6%Co balls. Although friction coeYcients could not be correlated to the amorphous BN contents, it was noted that low combined ball and disc wear rates were found for coatings containing large amounts of BN. Coatings with B/Al ~5 and elastic moduli of 290 GPa showed two orders of magnitude better sliding wear resistance against WC/6%Co balls compared to commercially available TiN.

Evaluating the microstructure and performance of nanocomposite PVD TiAlBN coatings

Abstract Physical vapour deposition (PVD) TiAlBN coatings can exhibit excellent wear resistance, with optimised coating compositions demonstrating a 150% increase in lifetime compared to standard TiAlN coatings in wet-cutting drill tests. For various TiAlBN compositions deposited by electron-beam evaporation, the stoichiometry and relative phase composition were determined using X-ray photoelectron spectroscopy (XPS) and the microstructure was examined by transmission electron microscopy (TEM). Al was found to substitute for Ti into the cubic TiN structure. In accordance with their position in the Ti(Al)BN phase diagram, all coatings exhibited a three-phase composition of (Ti,Al)N+BN+TiB 2 . The TiB 2 content was very small and the microstructure was effectively that of a (Ti,Al)N and BN dual-phase coating. Optimum drilling performance was obtained for a coating with a phase fraction of approximately 90% (Ti,Al)N and 10% BN. The microstructure can be described as nanocrystalline (Ti,Al)N grains separated by an intergranular amorphous BN phase, in which the average (Ti,Al)N grain size and grain separation was determined to be 26 and 3 nm, respectively. The presence of a compliant intergranular phase permits some degree of grain displacement under load, reducing the elastic modulus, leading to greater toughness and wear resistance.

Comparative tribology studies of hard ceramic and composite metal-DLC coatings in sliding friction conditions

The design of anti-friction coatings able to perform well in different wear conditions without lubricants requires a combination of adequate hardness and toughness, good adhesion, a low friction coefficient and a low wear rate. Recently introduced metal-diamond-like carbon (DLC) coatings can be a major step towards the achievement of such combination. These coatings consist of an amorphous a.CH matrix with the possible incorporation of metal, metal carbide and/or graphite grains. Previous studies of Tix%-DLC coatings deposited on Ti-TiN-TiCN-TiC supporting interlayers showed their good protective properties against abrasive, impact and single scratch wear. Here, ball-on-disk experiments are carried out to investigate these coatings in conditions of sliding wear against steel and WC balls. A comparison of the results for Tix%-DLC and ceramic. TiN, TiCN and CrN coatings shows particularly low friction coefficients (below 0.2) and low normalized wear rates for the Tix%-DLC coatings. The results reveal a corr...

X-Ray diffraction investigations of magnetron sputtered TiCN coatings

X-ray diffraction measurements were performed on titanium carbonitride coatings produced in an electron enhanced, unbalanced magnetron system. The films were deposited onto AISI 316 stainless steel substrates with an argon background pressure in the range of 0.1 Pa. A gas mixture of nitrogen and acetylene was introduced in different ratios, which alters the carbon-to-nitrogen ratio in the coating. The composition of the film was determined by glow discharge optical emission spectroscopy and wavelength-dispersive X-ray analysis. The morphology of the films was determined by scanning electron microscopy. The peak positions, integral breadths and shape parameters were determined by X-ray diffraction, and the composition influence on these parameters was investigated. The applied X-ray diffraction methods were conventional Bragg-Brentano diffraction and also parallel beam low angle diffraction. Morphological and microstructural features are found which indicate that an increased carbon content in the film gives similar results to a decrease in bias voltage.

Active process control of reactive sputter deposition

Control of the density, composition, ionisation rate and arrival energy of species is one of the main objectives of research in the development of reactive magnetron sputtering. The deposition of the latest generation of multilayer and multi-component coatings requires the independent control of deposited flux parameters using fast response and reliable control systems. A review of recent advances in process control showed the potential of techniques such as unbalanced magnetron sputtering in a closed magnetic field configuration, thermionically enhanced deposition and closed loop control with optical gas and metal plasma emission monitoring. These techniques were combined in an active control system. Special software was used to provide automatic computer aided process control in the deposition of multilayer and multi-component coatings. The system has been evaluated on a range of refractory and DLC coatings and recommendations on process control are given.

The effect of boron additions on the tribological behaviour of TiN coatings produced by electron-beam evaporative PVD

Abstract TiN coatings containing small amounts of boron were synthesized by electron beam evaporating a Ti–B alloy material from a single crucible source into a plasma comprising Ar or an Ar/N2 mixture at a substrate temperature of 450°C. The influence of 2 at.% B on the structural, mechanical and tribological properties of electron-beam (EB)PVD coatings is described. All coatings, consisting of different phases according to their location in the ternary Ti–B–N phase diagram, showed dense structures. Glancing angle X-ray diffraction spectra revealed only the presence of the Ti and TiN phases, respectively, for coatings deposited in either Ar or Ar/N2 gas mixtures. The hardness and elastic modulus of the Ti(B)N coatings increased linearly with increasing nitrogen content. Slightly overstoichiometric Ti(B)N coatings (Ti0.44B0.02N0.54) showed wear rates in reciprocating sliding wear tests against cemented tungsten carbide balls two orders of magnitude lower compared to a similar commercial EBPVD TiN coating.

Deposition and characterisation of TiAlBN coatings produced by direct electron-beam evaporation of Ti and Ti-Al-B-N material from a twin crucible source

Abstract TiAlBN coatings were synthesised by evaporating Ti and hot isostatically pressed TiAl0.68B3.67N2.44 material from a twin crucible source in an Ar plasma at 450 °C. A combination of optical emission spectroscopy and partial pressure control was utilised to control the Ti and TiAl0.68B3.67N2.44 evaporation rates and hence the Ti-Al-B-N composition in the coating. The goal of this investigation was to study the influence of composition on the structural and mechanical properties of these coatings. The influence of the Ti/B ratio (in the range of 0.34–2.31) in TiAlBN coatings containing approximately 12 at. % Al and 40 at. % N (calculated from glow discharge optical emission spectroscopy data) is described. All coatings, consisting of two hard phases TiB2 and TiN, and a soft hexagonal BN phase, showed dense structures and a decrease in roughness values with increasing Ti/B ratios. The hardness and elastic modulus increased with increasing Ti/B ratios from 0.34 to 2.31 up to 37 GPa and 320 GPa, respectively. Coatings with Ti/B = 1.73–2.31 showed a twofold increase in wet cutting performance compared to commercially available TiN and TiAlN. However, due to the rather low amount of Al in the TiAlBN coatings no significant improvement in dry cutting compared to TiN was observed.