Carburizing and nitriding as surface pre-treatment of PVD coating for gears application

Among the different treatments that can be carried out to locally improve the mechanical behaviour of gears a combination of case hardening followed by PVD coatings (duplex treatment) seems to give promising results in terms of surface hardness, residual stress profile and fatigue resistance. In particular considering the carburizing and the nitriding treatments they can be both aimed, in the same way than the surface coatings, to introduce a different mechanical behaviour between surface and core in order to improve life, reliability and load capacity of the treated component. This is fundamental for gears whose damage is mainly related to contact fatigue, fatigue at the tooth root and pitting on the tooth flank [1-3]. The need of optimising the surface material in order to delay the progressive deterioration of the components due to wear, fatigue or contact fatigue mechanisms, often worsened by the presence of hostile environments, explains the increasing attention on different coating technologies [5-7], In particular, considering the PVD coatings, chemical composition of the surface deposited film, coating thickness, hardness, adhesion with the substrate material and plastic deformation of the substrate material have an important influence on the damage mechanism affecting the coated component. Although hard PVD coatings are well known for improving friction and resistance to wear and corrosion, their tribological performance is often limited by elastic and plastic deformation of the substrate, which can allow to coating failures [12]. The emergence of the duplex treatments, consisting in the sequential application of two o more established surface technologies, has represented a novel approach to the achievement of enhancing coating properties. Duplex treatments, comprising a nitriding treatment followed by the deposition of a hard PVD coating, have been proven to be successful in increasing wear, thermal fatigue and corrosion resistance and the load carrying capability of different steel substrates [13-16]. By increasing the hardness of the substrate, for instance using a nitriding case, often provides a suitable load support for PVD coatings so that superior wear resistance can be achieved. The high values of hardness related to the thermochemical treatment, further enhanced by the introduction of the ceramic coating characterized by a strong difference in coefficient of thermal expansion with respect to the substrate material, affects the surface level of compression residual stress data [21-23], Therefore the residual stress gradient must be evaluated when a prediction of the gear life is requested: in fact the residual stress distribution affecting the nucleation of the fatigue cracks is a factor able to control the gear performance. Starting from such considerations, this work is focused on the microstructural (fig.2, fig. 4) and mechanical characterization (nanohardness and fatigue behaviour) of a CrN coating, about 5 μm thick, deposed by PVD technique on two different steels: a carburizing 16MnCrS5 steel grade and a nitriding 42CrMo4 steel grade (Table I). CrN films were deposited by means of the standard cathodic. arc using an industrial devices. Before coating the fatigue specimens (Fig. 1) were polished with a 3 μm diamond suspension and then ultrasonically cleaned. On the basis of published works [11] it is known that, in the case of nitrided substrates, the adhesion with the PVD coating is enhanced by the presence of Feα(N) structure while ε-Fe2-3N or γprime;-Fe 4N ones are detrimental. For such a reason a NITREG treatment was executed on the 42NiCrMo4 steel grade with the purpose of producing a low white layer, further reduced, before the coating deposition step, by means of a mechanical samples polishing targeted to remove the superficial brittle and porous layers. A short ion cleaning executed with Ar was carried out before the beginning of the coating deposition phase. The steel temperature was kept constant at 180°C with an initial peak of 210°C acting for about 2 minutes, independently from the type of substrate considered. Microhardness profiles were measured both on uncoated and on coated samples in order to determine both the thickness of the carburized and nitrided layers and the effect of the thin film deposition process (fig. 3). The coating nanohardness data were also measured by the depth sensing technique using a Fisherscope H100 nanoindenter operating by a computer controlled stress limited device and equipped with a Vickers indenter. X-ray diffractometry (XRD) was used to identify the chemical coating composition (fig. 2) and to measure the residual stresses induced from the sample's process route including the coating step. XRD with Bragg Brentano geometry were performed with a Philips PW 1830 instrument with a goniometer PW 3020 and a control unit Philips PW 3710 (Cu K α radiation, scan rate 1° /min). Surface residual stresses were detected using Cu Kα radiation by means of a Italstructure Stress X3000 diffractometer. The stresses (-120±25 MPa after carburizing; -580±40 MPa after nitriding; -1870±87 MPa after carburizing + PVD and -2350±114 MPa after nitriding + PVD) were calculated using the sinj2 method and adapting the elastic modulus value obtained by nanoindentation measurements and assuming a Poisson ratio of 0.2, value usually taken as a reference when ceramic CrN or Cr(C,N) thin films are considered. Using a rotating bending machine fatigue tests were carried out both on case hardened samples and nitrided plus PVD coated specimens (fig. 1). Experiments were executed at room temperature, in air, at a test frequency of 33 Hz using a sinusoidal load wave form and a load ratio (minimum to maximum load) of R=0. The stress level at which specimens can run without occurrence of failure after 3 · 106 stress cycles was chosen as the fatigue limit. Results of the fatigue tests were analysed according to the stair-case up and down method (Table II). The presence of the PVD film is responsible for a light increase in the fatigue resistance both for the carburized samples and for the nitrided ones. Fatigue nucleation sites resulted affected from the presence of PVD coating only in the case of carburized substrate: the high residual stress level characterizing the ceramic coating excludes the surface as nucleation zone and moves it at the interface with the steel material (fig. 5). No change in the nucleation areas were observed in the nitrided specimens or in the nitrided and coated samples (fig. 6) where the weak points resulted the non metallic inclusions inside the substrate material.