M. Andritschky

Effects of deposition temperature and thermal cycling on residual stress state in zirconia-based thermal barrier coatings

Abstract Advanced ceramic multilayered coatings are commonly used as protective coatings for engine metal components to improve performance, e.g. thermal barrier coatings (TBCs). Zirconia-based TBCs were produced by plasma spraying process and characterized in terms of microstructure, porosity, elastic modulus, adherence and residual stresses. In this contribution the residual stresses in multilayered coatings applied on Ni based superalloys for use as thermal barrier coatings were studied both by numerical modelling and experimental stress measurement. The thermal residual stresses generated during the spraying process of duplex TBCs were simulated by using an heat transfer finite element program and an elasto-plastic biaxial stress model. The TBC system was subjected to different thermal cycling conditions (maximum temperature, heating up and cooling down rates, dwell time at maximum temperature, etc.). The stress distribution within the TBC was also modelled after thermal cycling. The stress state in the as-deposited and in thermally cycled coatings was verified using an X-ray diffraction technique. The measurements were in good agreement with the residual stress modelled calculations. It was observed that the residual stresses were dependent on the thermal history of the TBC (as-deposited and thermally cycled). It is proposed that thermal cycling allowed the stresses to relax by microcracking and creep mechanisms at high temperature such that on cooling down to room temperature, an in-plane biaxial compressive stress will arise on the zirconia top coating due to the difference on the coefficients of thermal expansion between the metallic substrate and ceramic coating material.

Microstructure of CrN coatings produced by PVD techniques

Abstract The surface of machine parts and moulds in plastic transformation processes may be exposed to corrosive agents evaporated from the plastics at elevated temperatures and to the abrasive action of hard fibres in the case of reinforced plastics. Nitrides of transition metals can be wear resistant but also resistant to many chemically aggressive environments. The microstructure and mechanical properties of chromium nitride based coatings were studied in order to analyse the potential of this material to be used as a protective coating of the surface of injection moulding or extruding machines. Chromium nitride based hard materials were deposited by reactive magnetron sputtering onto stainless steel substrates in the form of homogeneous coatings. The coating microstructure and morphology were studied by X-ray diffraction (XRD), optical and atomic force microscopy (AFM). The mechanical properties of these coatings were studied by scratch and nano-indentation testing and residual stress measurements. The coatings can be strongly textured, if unbiased during deposition. Optical image analysis revealed the density and size distribution of defects. Atomic force microscopy was used to identify these defects as pinholes, particulates and grains growing with different growth direction. The coating residual stress depends, of course, on the strain within the growing grains and therefore on the deposition conditions. But the residual stress was also influenced by the coating morphology, namely the coating density. The Youngs moduli of CrN-PVD coatings depend on the measuring direction as well as on the coating morphology. Normal to the coating surface a Youngs modulus of about 400 GPa can be found. Parallel to the substrate surface, the E -modulus was very much affected by the existence of voids or loosely packed CrN and varied between 100 and 300 GPa. Some interesting features were found such as the case of open porosity being frequently found on coatings with higher density and lower effective elastic modulus and coatings with lower density and higher effective elastic modulus do not necessarily show open porosity.

Corrosion of CrN and TiAlN coatings in chloride-containing atmospheres

Abstract Chromium and titanium–aluminium nitrides were deposited, by physical vapour deposition techniques, on stainless steel substrates and their corrosion behaviour was studied in two different environments: a gaseous environment containing HCl at a temperature of 350°C and a 1 M HCl aqueous solution environment, at room temperature. X-ray photoelectron spectroscopy was used to study the mechanism of the reactions that occurred in the gaseous environment. This analysis shows an oxidation of the surface caused by the substitution of nitrogen by oxygen. Open circuit potential and potentiodynamic polarisation measurements were performed in the aqueous solution environment. The aqueous corrosion behaviour of the nitride coatings is strongly dependent on the microdefect density of the coating.

Thermal oxidation of Ti1-xAlxN coatings in air

Abstract (Ti,Al)N coatings prepared by combined DC and RF magnetron sputtering were annealed in air at temperatures between 500 and 900 °C, in order to obtain information regarding the oxidation behaviour. The depth concentration profile of the oxidized layers was measured by Rutherford Backscattering Spectrometry (RBS). During the heat treatment at 500 °C, the Ti 0.35 Al 0.65 N coating forms a Ti and Al mixed oxide with about 10 at% of nitrogen. After the annealing at 600 °C of Ti 0.62 Al 0.38 and Ti 0.35 Al 0.65 N coatings, the nitrogen amount disappears and the oxide layer is still homogeneous. At temperatures between 750 and 900 °C, a two-layer structure is formed, consisting generally in a protective superficial layer of Al 2 O 3 with traces of Ti, followed by a titanium-rich zone. The Ti 0.35 Al 0.65 N system showed a slightly higher oxidation resistance than the Ti 0.62 Al 0.38 N one. On the other hand, the Al-rich coating, Ti 0.19 Al 0.81 N, revealed the worst oxidation resistance, similar to the AlN coating, and the oxide layer is always homogeneous.

Oxidation resistance of (Ti,Al,Zr,Si)N coatings in air

We prepared TiN-based multicomponent hard coatings by combined d.c. and r.f. magnetron sputtering with different contents of Ti, Al, Zr and Si on high speed steel substrates at 300°C. These coatings, with thicknesses ranging from 1 to 3 μm, were annealed in air at temperatures between 500 and 850°C in order to obtain information on their oxidation behaviour. The composition-depth profiles of heat-treated coatings were then measured by Rutherford backscattering spectrometry (RBS). The results show that the introduction of aluminium improves the oxidation resistance in all cases. During the heat treatment of Ti0.62Al0.38N at 600°C, a mixed oxide of Ti and Al is formed. At temperatures between 700 and 850°C, a protective superficial layer of Al2O3 with traces of Ti is formed, which is followed by an aluminium-depleted zone. However, for Ti0.57Al0.38Zr0.05N coatings, no protective Al2O3 layer was found on the surface after heat treatment. At 600°C, the oxidation resistance of Ti0.57Al0.38Zr0.05N is similar to that of (TiAl)N, but is about 30 times poorer at 700°C due to the absence of the protective Al2O3 layer. The Ti0.62Al0.26Si0.12N system shows a slightly lower oxidation resistance than (TiAl)N (Kp = 9.0 × 10−12 kg2 m−4 s−1 and Kp = 6.4 × 10−12 kg2 m−4 s−1 at 800°C respectively). It also forms a two-phase scale as in (TiAl)N, but the amount of Ti in the Al-rich outer layer is about 10 at.%, instead of 4at.% found in the (TiAl)N system. In the temperature range 700–850°C, oxidation is thermally activated with activation energies of 187 kJ mol−1 and 296 kJ mor−1 for (TiAl)N and (TiAlSi)N coatings respectively.

Corrosion of TiN, (TiAl)N and CrN hard coatings produced by magnetron sputtering

Abstract Metallic components like moulds, dies and machinery can be subjected to intensive degradation during plastic transformation processes, namely when working with fibre filler materials and plastics which release F, S or Cl during transformation. The degradation is attributed to the combined erosive and abrasive wear by the filler material and corrosive attack of agents. This degradation reduces the lifetime of the components considerably and has a direct impact on process productivity and surface finish of the final products. Nitride-based hard coatings like TiN, (TiAl)N, BN, etc. have proved their capability to increase tool lifetime when exposed to abrasive and corrosive environments found in plastic transformation processes (halogenated polymers, acrylics, polyesters, fibre reinforced plastics, etc.). Within the frame of this work we produced TiN, (TiAl)N, CrN hard coatings, with and without a metallic interlayer, by dc and rf reactive magnetron sputtering, with a thickness of about 2 μ m. The aqueous corrosion behaviour of the coatings was studied in saline and acidic environments by potentiodynamic and open circuit potential (OCP) measurements. The oxidation resistance during annealing in air was also studied. In saline (NaCl 9%) and acid (HCl 3.4%) environments we found that a metallic interlayer of Ti or Cr in the case of TiN–(TiAl)N-coated samples and CrN-coated samples, respectively, generally improve the corrosion resistance. Best results for all tested nitride coated samples were obtained for the Ti 0.27 Al 0.73 N coating. The OCP vs. Saturated Calomel Electrode (SCE) (60 min) measurements indicated that most samples were nobler than the un-coated substrate. The mentioned potentials depend on the deposition conditions and the film microstructure. Most of the coatings lose some of their protective capabilities after an high temperature annealing. In contrast to the Ti-based hard coatings, the corrosion resistance of CrN is improved by a 800°C annealing treatment in air.

Characterisation of ZrO2 films prepared by rf reactive sputtering at different O2 concentrations in the sputtering gases

Abstract Zirconium oxide (ZrO 2 ) films have been prepared by rf reactive magnetron sputtering at different O 2 concentrations in the mixture sputtering gases. The films have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and optical spectroscopies. The influence of O 2 concentration in the sputtering gases on the microstructure, residual stress and optical properties of the films has been studied. Also, the effect of loose packing structure caused by the high O 2 gas concentration on the deposition rate has been discussed.

Zinc oxide films prepared by dc reactive magnetron sputtering at different substrate temperatures

Abstract The properties of zinc oxide films prepared by dc reactive magnetron sputtering were studied over a range of the substrate temperatures (room temperature to 450°C). The dependence of the structure of the films on the substrate temperature was studied using scanning electron microscopy and X-ray diffraction. The films had a preffered orientation along the (002) crystal plane at room temperature (50°C), a random orientation at 200–300°C and again a preferred orientation along the (002) crystal plane at 350–450°C. The grain size increased as the substrate temperature was raised. In addition, the optical and electrical properties have also been investigated.

Performance of chromium nitride and titanium nitride coatings during plastic injection moulding

Monolithic coatings of chromium nitride, titanium nitride and multilayer titanium/chromium nitride coatings were produced by r.f. and d.c. reactive magnetron sputtering in order to determine their potential to be used as protective coatings for machinery parts of plastic injection moulding or extruding machines. The tribological and mechanical behaviour of these coatings were studied. Monolithic coatings showed lower wear rates, measured by pin-on-disc experiments, when compared with multilayer coatings. The oxidation of the surface was also lower in monolithic coatings. The performance of the coatings during plastic processing was tested in a dye fitted to an injection moulding machine and using glass reinforced thermoplastic. The wear rates of the nitride-based coatings during plastic processing was more than two orders of magnitude better then some traditional methods of protecting the surfaces such as hardening the steel by heat treatment, electrodepositing hard chromium or nitriding the steel surface. The physical vapour deposition coatings also showed higher corrosion protection during plastic processing tests.

Performance of chromium nitride based coatings under plastic processing conditions

Chromium nitride based coatings were produced in the form of monolithic and multilayer coatings, by DC and RF reactive magnetron sputtering. These coatings were deposited onto stainless steel and tool steel substrates. Chromium nitride coatings have proved to be wear and corrosion resistant. The combination of these characteristics was necessary to protect surfaces during plastic processing. In order to select the best coatings, some mechanical and tribological tests were performed. Hardness and Youngs modulus of the produced coatings were measured by nano-indentation, adhesion was assessed by scratch test experiments and wear was evaluated by performing pin-on-disc experiments. The behaviour of these coatings in chemically aggressive environments was studied by different corrosion experiments. The behaviour of these coatings in thermoplastic processing equipment was also studied. The abrasive and corrosive wear of chromium nitride based coatings was assessed under relatively high pressures and temperatures and was compared with the surface wear of hardened tool steel substrate by heat treatment, nitrided by chemical-heat treatment, or protected by hard chromium coating deposited in a galvanic way. Monolithic coatings were harder, and had higher Young modulus and adhesion when compared with multilayered coatings. Their wear resistance, measured by pin-on-disc test, was between two and three orders of magnitude higher, but multilayer coatings showed significantly better corrosion resistance. In general, monolithic and multilayer coatings have significantly better behaviour when compared with traditional processes of protecting functional surfaces for plastic transformation equipment. The measured wear of the best performance obtained by chromium nitride based coatings can be more than two orders of magnitude lower than nitrided surfaces or hard chromium coated samples.