TRIBOLOGICAL PROPERTIES AND WEAR MECHANISM OF HARD COATINGS

In the modern technology, tribologically suitable components and devices are important to increase the energy efficiency. It is possible when one can reduce the friction coefficient and wear of sliding components. The economic effectiveness can be achieved by better tribological system and therefore research in tribology is aimed at minimizing the energy losses resulting from friction and wear. In this view, hard coatings deposited by physical vapor deposition (PVD) are adequate solutions for increasing the work efficiency, lifetime of tools and components. The present thesis deals with hard tribological materials γ-TiAl and coatings such as TiAlN, CrN/NbN superlattice, diamond like carbon (DLC) and nanocrystalline diamond nanowire (DNW) films. Various characterization techniques were used to study morphology, microstructure and chemical state of the materials. The thesis describes tribological properties of above mentioned hard coatings sliding against 100Cr6 steel, Al2O3 and SiC balls. It also describes friction and wear based on classified mechanisms and outlines material properties that influence the performance of sliding surfaces. Traditionally wear is associated with friction and wear mechanisms are classified as adhesion, abrasion, erosion, fatigue and oxidational. Mechanical and tribological properties of γ-TiAl alloy, TiAlN, CrN, carbon based coatings of DLC and nanocrystalline DNW were reviewed. The importance of such hard coatings and critical application in machine and industries are highlighted. Moreover, tribological properties and evaluation of wear mechanism is introduced in the respect of microstructure and chemical behavior of the sliding interfaces. Various wear mechanism with different combination of sliding surfaces such as hard coating/soft ball and soft ball/hard coating is reviewed in order to understand the wear mechanism. The fundamentals of some of the characterization techniques used to study the mechanical, tribological, morphological, structural, and chemical properties of the coating and wear track is introduced. Instrumented micro-indentation technique have been used to characterize the mechanical properties namely hardness and elastic modulus of γ-TiAl alloy. The recorded indentation curves and the related energetic properties were analysed in order to compare the Attaf energetic approach and Oliver-Pharr method. Moreover, tribological behavior of γ-TiAl alloy was studied by sliding against 100Cr6 steel, SiC and Al2O3 balls as counterbodies for friction pairs. The friction coefficient and wear rate was found to high when γ-TiAl alloy slides against Al2O3 and SiC balls. However, these values were less while sliding against steel ball. The wear mechanism is explained by the sliding combination of harder/harder system such as SiC/γ-TiAl, and Al2O3/γ-TiAl alloy. However, steel/γ-TiAl alloy acts as softer/harder sliding combination. Tribological behavior of TiAlN coating were studied by sliding against 100Cr6 steel, SiC and Al2O3 as counterbodies for friction pairs. Two distinct types of wear modes such as oxidational and plastic deformation are investigated. It is shown that wear of metal debris tribochemically reacted with moisture available in ambient atmosphere and metal oxide formation which leads oxidational wear in TiAlN/steel sliding pair. In TiAlN/SiC sliding pair, low friction coefficient is measured and this is attributed to the formation of lubricious composite tribofilm. In contrast, TiAlN/Al2O3 pair shows high friction coefficient and wear mechanism is governed by plastic deformation. CrN/NbN superlattice coating sliding against 100Cr6 steel, SiC and Al2O3 ball as counterbodies for friction pairs was investigated. The value of friction coefficient and wear rate was lowest ~0.01 and 2.6×10–7 mm3/Nm, respectively, when coating slides against Al2O3 ball. In contrast, friction coefficient and wear rate is increased while sliding with steel and SiC balls. It is observed that the deviation in friction coefficient is described by mechanical and chemical properties of these balls. In this respect, hardness of Al2O3 and SiC ball is comparable but significant deviation in friction coefficient is observed. This is related to oxidation resistance of balls which is high for Al2O3 compared to SiC as evident by Raman analysis of the wear track. However, steel ball shows oxidational wear mechanism against CrN/NbN superlattice coating. The tribological properties of DLC and nanocrystalline DNW films were investigated. A friction mechanism based on surface chemistry and mechanical properties of sliding interfaces such as DLC/100Cr6 steel, DLC/SiC and DLC/Al2O3 is studied. In DLC film, the high friction coefficient is governed by surface roughness of the sliding interfaces during initial sliding passes. However, for longer sliding cycles, the sliding interfaces get smoothened and magnitude of friction coefficient is reduced. Under these experimental conditions, carbonaceous transferlayer forms on the ball sliding surface. Nanocrystalline DNW films was deposited in N2 enriched microwave plasma enhanced chemical vapor deposition (MPECVD) system. As-deposited DNW film was treated in O2 plasma which resulted in chemical and microstructural modification. The sheath of the nanocrystalline DNW is chemically constituted by amorphous carbon (a-C) and graphite (sp2C-C) like bondings. However, nanowires transformed into ultra- small spherical grains after the O2 plasma treatments. In this condition, a-C and sp2C-C fraction get significantly reduced due to plasma etching. Oxidation and formation of functional groups increases on the surface and inside the wear track. The friction coefficient of O2 plasma treated film showed super low value of ~0.002 with exceeding high wear resistance of 2×10–12mm3/Nm. Such an advance tribological properties is explained by passivation of covalent carbon bonding and transformation of sliding surfaces by van der Waals and hydrogen bondings. High surface energy and the consequent superhydrophilic behavior of film attributed to the formation of an adsorbate layer of above mentioned functional groups which acts as a lubricant

A new test device for the study of metal wear in conditioned granular soil used in EPB shield tunneling

The wear phenomenon evaluation in EPB shield tunneling machines is not a simple issue, as a large number of parameters are involved, such as soil and tool material properties, soil conditioning and pressure in the bulk chamber. The evaluation of the influence of these parameters and predicting this influence is a complex task and the research has proposed different test procedures and approaches. In this paper a new procedure for testing wear of tools with an innovative concept and design is presented. The experimental results obtained using conventional steel and hard material tools, tested with natural and conditioned soils, are discussed. The outcomes show the feasibility of the proposed procedure and the quality of the measurements that can be obtained using the proposed wear tool shape

FAILURE MODES OF PVD COATINGS IN MOLTEN AL-ALLOY CONTACT

This paper deals with a study of the failure mode of thin PVD coatings in alternated contact with molten aluminum alloy. CrN and ZrN monolayer coatings deposited through cathodic arc evaporation were used. The coatings morphology was assessed by SEM and their mechanical properties evaluated by nanohardness test performed at room temperature. An experimental test rig which cyclically immerses coated steel samples in molten Al-alloy and in a cooling bath was applied. The thermal gradient from the coating to the steel core was exalted by internal cooling channels placed in the internal cavity of samples. Periodical SEM inspections were performed to assess the damaging levels introduced by the test and to study the related decrease of substrate protection capability. Descriptions and interpretations of the damages evolutions were derived. The main conclusions achieved are that both coatings suffered by the formation of corrosion pits, which were due to a corrosion attack of the steel substrate localized at coating defects sites. In particular, at pores locations the corrosion was fast, whereas at droplets sites it required a certain incubation time. Once corrosion pits were formed they exhibited an initial tendency to expand laterally, but they did rapidly stabilize in terms of lateral dimensions. Later on two different failure modes acted in ZrN and in CrN. Extended delamination due to a marked mismatch of mechanical properties between the coating and the steel substrate developed in ZrN. On the contrary, thermal cracking due to lower hardness levels developed in CrN, but with limited delamination. Accordingly the steel substrate protection capability was evaluated to be higher in CrN than in ZrN. Keywords: PV

Parameters Optimization and Repeatability Study on Low-Weldable Nickel-Based Superalloy René 80 Processed via Laser Powder–Bed Fusion (L-PBF)

This work aims to investigate the processability of René 80 via laser powder–bed fusion (L-PBF). René 80 is a poorly weldable Ni-superalloy, currently processed via investment casting to fabricate turbine blades working at an operating temperature of about 850 °C. The L-PBF parameters optimization aims to increase part integrity and enhance processing repeatability. This part was tackled by creating a complete design of experiments (DOE) in which laser power, scan speed and hatching distance were varied accordingly. Optimizing the abovementioned parameters minimized the crack density and pore area fraction. Hence, five parameter sets leading to a crack density lower than 100 µm/mm2 and a pore fraction between 0.045% and 0.085% were selected. Furthermore, the intra-print repeatability was studied by producing three specimens’ repetitions for each optimal set of parameters in the same build. The porosity value obtained was constant among repetitions, and the crack density (around 75 µm/mm2) had a slight standard deviation. The third step of the research assessed the inter-prints repeatability by producing a replica of the five selected parameter sets in a different build and by comparing the results with those studied previously. According to this latter study, the porosity fraction (ca. 0.06%) was constant in intra- and inter-print conditions. Conversely, crack density was lower than 100 µm/mm2 only in three sets of parameters, regardless of the intra- or inter-build cross-check. Finally, the best parameter set was chosen, emphasizing the average flaw fraction (least possible value) and repeatability. Once the optimal densification of the samples was achieved, the alloy’s microstructural features were also investigated

Effect of Cold Rolling on Microstructural and Mechanical Properties of a Dual-Phase Steel for Automotive Field

A new advanced dual-phase (DP) steel characterized by ferrite and bainite presence in equal fractions has been studied within this paper. The anisotropy change of this steel was assessed as a progressively more severe cold rolling process was introduced. Specifically, tensile tests were used to build a strain-hardening curve, which describes the evolution of this DP steel's mechanical properties as the thinning level increases from 20 to 70% with 10% step increments. As expected, the cold rolling process increases mechanical properties, profoundly altering the material's microstructure, which was assessed in depth using Electron Backscatter Diffraction (EBSD) analysis coupled with the Kernel Average Misorientation (KAM) maps. At the same time, the process strongly modifies the material planar anisotropy. Microstructural and mechanical assessment and the Kocks-Mecking model applied to this steel evidenced that a 50% strain hardening makes the DP steel isotropic. The material retains or resumes anisotropic behavior for a lower or higher degree of deformation. Furthermore, the paper evaluated the forming limit of this DP steel and introduced geometric limitations to testing the thin steel plates' mechanical properties

Hardness Evolution of Solution-Annealed LPBFed Inconel 625 Alloy under Prolonged Thermal Exposure

Thanks to its high weldability, Inconel 625 (IN625) can be easily processed by laser powder bed fusion (LPBF). After production, this alloy is typically subjected to specific heat treatments to design specific microstructure features and mechanical performance suitable for various industrial applications, including aeronautical, aerospace, petrochemical, and nuclear fields. When employed in structural applications, IN625 can be used up to around 650 °C. This limitation is mainly caused by the transformation of metastable γ″ phases into stable δ phases occurring under prolonged thermal exposure, which results in drastically reduced ductility and toughness of the alloy. Because the microstructure and mechanical properties change during thermal exposure, it is essential to study the material simulating possible service temperatures. In the current study, LPBFed IN625 samples were solution-annealed and then subjected to thermal exposure at 650 °C for different times up to 2000 h. The characterization focused on the evolution of the main phases, γ″ and δ phases, and their influence on the hardness evolution. The microstructure and hardness of the heat-treated LPBFed IN625 samples were compared with data related to the traditionally processed IN625 alloy (e.g., wrought state) reported in the literature

Study of the Microstructure and Cracking Mechanisms of Hastelloy X Produced by Laser Powder Bed Fusion

Hastelloy X (HX) is a Ni-based superalloy which suffers from high crack susceptibility during the laser powder bed fusion (LPBF) process. In this work, the microstructure of as-built HX samples was rigorously investigated to understand the main mechanisms leading to crack formation. The microstructural features of as-built HX samples consisted of very fine dendrite architectures with dimensions typically less than 1 µm, coupled with the formation of sub-micrometric carbides, the largest ones were mainly distributed along the interdendritic regions and grain boundaries. From the microstructural analyses, it appeared that the formation of intergranular carbides provided weaker zones, which combined with high thermal residual stresses resulted in hot cracks formation along the grain boundaries. The carbides were extracted from the austenitic matrix and characterized by combining different techniques, showing the formation of various types of Mo-rich carbides, classified as M6C, M12C and MnCm type. The first two types of carbides are typically found in HX alloy, whereas the last one is a metastable carbide probably generated by the very high cooling rates of the process

Low-Power Laser Powder Bed Fusion Processing of Scalmalloy®

Among recently developed high-strength and lightweight alloys, the high-performance Scalmalloy(®) certainly stands out for laser powder bed fusion (LPBF) production. The primary goal of this study was to optimize the Scalmalloy(®) LPBF process parameters by setting power values suitable for the use of lab-scale machines. Despite that these LPBF machines are commonly characterized by considerably lower maximum power values (around 100 W) compared to industrial-scale machines (up to 480 W), they are widely used when quick setup and short processing time are needed and a limited amount of powder is available. In order to obtain the optimal process parameters, the influence of volumetric energy density (VED) on the sample porosity, microstructure and mechanical properties was accurately studied. The obtained results reveal the stability of the microstructural and mechanical behaviour of the alloy for VEDs higher than 175 Jmm(−3). In this way, an energy-and-time-saving choice at low VEDs can be taken for the LPBF production of Scalmalloy(®). After identifying the low-power optimized process parameters, the effects of the heat treatment on the microstructural and mechanical properties were investigated. The results prove that low-VED heat-treated samples produced with an LPBF lab-scale machine can achieve outstanding mechanical performance compared with the results of energy-intensive industrial production