Heinz-Joachim Spies

Plasma nitriding of stainless steels at low temperatures

Abstract To avoid the drop in corrosion resistance of stainless steels in conventional nitriding (precipitation of CrN), low-temperature techniques like ion implantation, plasma immersion ion implantation (PIII, PI3) and low-temperature plasma nitriding were developed. In this investigation, four stainless-steel grades (ferritic: X6Cr17, austenitic–ferritic: X2CrNiMoN22.5.3, austenitic: X8CrNiTi18.10 and X5CrNi18.10) were plasma-nitrided between 250 and 500°C. Nitrogen-enriched layers with a high nitrogen content were produced, leading to a significant increase in surface hardness. X-ray diffraction indicated that CrN did not precipitate if treatment temperatures did not exceed 400°C. ‘Expanded austenite’ formed in the austenitic and duplex steels and E-nitride (Fe2N1−x) in the ferritic steel. The optically visible structure of the nitrided cases is comparable with that of the PIII layers, with higher charging densities being possible in the plasma nitriding. Also, in comparison to conventional ion implantation, large charges and parts with complicated shapes can be treated.

Wear resistance of prenitrided hardcoated steels for tools and machine components

Abstract Hardened and tempered low-alloy steel 31CrMoV9 and the high-alloy tool steels S 6-5-2 and X155CrMoV121 were nitrided to form a varied structure of the substrate for the subsequent hardcoating. The tool steels were nitrided and hardcoated in a continuous process in a modified commercial PVD plant. The duplex treatment of the low-alloy steel was realized by separate nitriding and hardcoating in different plants. The TiN and CrN were deposited with a thickness of approx. 3 μm by hollow cathode discharge evaporation. The composition and structure of the nitrided case, the interstage treatment before deposition, as well as the deposition parameters influence the properties of the composite. The adhesion can be improved essentially by prenitriding and deposition of a gradient interlayer system. The resistance of the tool steels to metal cutting and forming increases due to the production of an application-specific duplex layer. The resistance to sliding wear and contact fatigue was investigated on various duplex-treated low-alloy steel by nitriding the substrate. Whereas the nitrided case has a very high influence on the contact fatigue limit, the hardcoating reduces the wear by sliding and abrasion, which is of special interest for machine components with higher slip.

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.

Adhesion and wear resistance of nitrided and TiN coated low alloy steels

Abstract Hardened and tempered low alloy steel grade 31CrMoV9 was gas and plasma nitrided to form a varied structure of the substrate for a subsequent TiN hardcoating. The process parameters of the nitriding were modified to form a sufficiently supporting diffusion layer with a thickness of more than 0.5 mm and a defined structure of the surface. Hardened cases without compound layer were formed using more step technologies in the nitriding process. The TiN coating was deposited by hollow cathode discharge evaporation. The composition and structure of the nitrided case, the mechanical pretreatment, the plasma assisted treatment directly before deposition as well as the deposition parameters influence the properties of the duplex treated steel. The adhesion can be improved essentially by an interlayer system TiTiN x TiN. Special consideration was given to the investigation of the adhesion and wear resistance of the composites. The wear tests include investigations of the sliding and abrasive wear. The results of testing rolling butt contact shows that a TiN layer on nitrided steel has no essential influence on the fatigue limit.

Direct combination of plasma nitriding and PVD hardcoating by a continuous process

Abstract The properties obtained by the conbination of nitriding with hardcoating allows a function sharing among core material, hardened case and surface. Such combined properties are of interest for the application in complex stressed tools and machine components. The precondition of a succesful combination of nitrided steels with a hardcoating is especially the compatibility of the structure and the properties of the nitrided layer and the coating, but also the technological and economic aspects of the production of such composites. For example, a discontinuous combination of nitriding and hardcoating has some disadvantages, if no external intermediate treatment is provided. A commercial ion-plating equipment (TINA 900) was modified to realize a continuous process of pulse plasma nitriding and TiN hardcoating in one and the same equipment on various steel grades. Special consideration was given to the investigation of the composite properties such as adhesion, structure, hardness and residual stress as well as the tribological behaviour in dependence on the process parameters. The results of the plasma nitriding in the modified cold-wall reactor are comparable to those of nitriding in a hot-wall plant. The hardness and wear resistance increases. The hardcoating of the nitrided substrates leads to an increased adhesion by deposition of a thin Ti or Ti-TiNx intermediate layer.

Process technological aspects of the production and properties of in situ combined plasma-nitrided and PVD hard- coated high alloy tool steels

Abstract The successful combination of nitrided high alloy steels with a hard coating requires good compatibility between the structure and properties of the two layers. The technological and economic aspects of the production of such composites are also relevant. A commercial ion plating apparatus was modified to produce a continuous process of pulse plasma nitriding and TiN hard coating on tool steels in the same equipment. The nitriding depth was varied between 15 and 50 μm in process times with a maximum of 2 h. Special gas mixtures of argon, nitrogen and hydrogen were used to form a diffusion layer with a defined nitrogen content and hardness. The aim of the investigations was to determine the composite properties, such as adhesion, topography and composition (nitride layer, intermediate layer, TiN), hardness, residual stress and tribological behaviour, as a function of the process parameters and the interstage process control in the in situ combined process. With regard to the use of such duplex treated materials, tests for abrasive wear were carried out to characterize the tool behaviour. The properties, in particular the adhesion, can be improved depending on the conditions of pre-nitriding.

Duplex Treatment of Tools and Components: Previous or Subsequent Electron Beam Hardening of Thermochemically-Treated and PVD Hard-Coated Steels for Tools and Components

The tribological properties of tools and components are improved by wear resistant coatings deposited by thermochemical processing (nitriding, nitrocarburizing) or physical vapor deposition (PVD) technologies. The properties gradient between the very hard and brittle layer and the soft, ductile, but hardenable matrix material can be significantly improved by previous or subsequent electron beam hardening (EBH). The hardness of matrix material is increased by martensitic transformation up to a certain depth, which results in better load support for the hard coating layer. The influence of the tempering stability of the steels investigated and the chemical and structural stability of the coatings on the chosen sequence of treatments is discussed.

Deposition and properties of TiN/carbon multilayers for corrosion protection of steel

TiN/carbon multilayers for corrosion protection of steel were deposited using d.c. magnetron sputtering. Cross-section TEM pictures revealed a dense and fine-grained polycrystalline TiN structure whereas the carbon sublayers proved to be amorphous. Nodular growth defects connected with deep gaps were frequently observed; these are due to macroscopic carbon particles emitted from the graphite target. These defects are the most important cause for a corrosive attack. The pitting potential of the multilayers was increased in comparison with both TiN and carbon single layers.

Nitriding behaviour of the intermetallic alloy FeAl

Abstract An iron-based intermetallic FeAl alloy was gas nitrided under well-defined conditions, and the influences of the process parameters, i. e., time, temperature, and nitriding potential on the layer formation, were investigated. Microstructural, morphological, and chemical characterization of the nitride layer was performed by means of glow discharge optical spectroscopy, electron probe microanalysis, scanning electron microscopy, X-ray diffraction, internal-stress measurement, hardness–depth profiles, and indentation fracture mechanics. Pin-on-disk tests were carried out to investigate the load-bearing capacity and wear resistance of the nitride layers. The formation of hexagonal AlN during the nitriding treatment leads to an increase in hardness of about 920 – 980 HV 0.025 and to a significant improvement of the wear resistance. Additional annealing tests proved the thermal stability of the nitride layer up to 950 °C.

Control of plasma nitriding using oxygen probes

Abstract It has been shown by experiments, that a recently developed vacuum-tight oxygen probe enables the direct control of the oxidising action of plasma nitriding atmospheres. In connection with this, the ‘oxygen-sensitivity’ of steels with different chromium content has been investigated according to the variation of the hydrogen and amount of oxygen in the nitriding atmosphere. Threshold oxygen partial pressures for the transition from nitriding to oxidising were evaluated. These critical partial pressures decrease with increasing chromium content. The oxygen probe measurements confirm the influence of further parameters like temperature, pumping time, leakage and history of the facility (time interval under atmospheric pressure) on the oxygen partial pressure and thus on the nitriding results. Measuring the oxidising potential by an oxygen probe offers new possibilities in controlling and optimising plasma nitriding in commercial facilities. This is of special importance for the treatment of high alloyed steels and non-ferrous materials like aluminium.