Microstructural and elasto-plastic material parameters identification by inverse finite elements method of Ti(1-x) AlxN (0 < x < 1 sputtered thin films from Berkovich nano-indentation experiments

International audienceThe mechanical properties of Ti(1 − x)AlxN (0 b x b 1) films of different thicknesses deposited by r.f. reactive magnetron sputtering on Si b100N and high speed steel substrates have been investigated. The as-deposited coatings have been characterized by X-ray diffraction, atomic force microscopy, four-probe electric resistivity method,mechanical deflection of cantilever beams and Berkovich nano-indentation tests associated with inverse finite elements analysis. The coatings with x b 0.58-0.59 present a cubic structure whereas for x N 0.7 a hexagonal structure is observed. Between these two compositions cubic and hexagonal structures coexist. The roughness depends on the film thickness and on the Al content and a minimumassociated to a very fine microstructure is clearly observed in the two-phase coatings. The electric resistivity sharply increases as soon as the hcp structure appears (x ~ 0.6). The mean residual stresses are compressive, except for the AlN coating, and present a minimum at the neighborhood of x ~ 0.64 where a mixed structure is observed. The indentation modulus MbhklN and the Berkovich hardness HBbhklN greatly depend on the Al content and a progressive decreasing has been observed for 0.58 b x b 0.7. For the MbhklN evolution, a simple model taking into account the stiffness coefficients of TiN and AlN structures, the mean residual stress level and the variations of the lattice parameters in the two structure domains is proposed. Knowing the elastic properties of these films, inverse finite elements analysis of the indentation curves considering a simple isotropic linear elasto-plastic behavior allows, as a function of the composition, the yield stress σY and the linear hardening coefficient Hp⁎ to be estimated. σY and Hp⁎ are in the ranges 4.2 to 6.8 GPa and 60 to 400 GPa, respectively. The maximum value of Hp⁎/σY which characterizes the ability of these coatings to exhibit plastic strain hardening is maximum for x = 0.5 and 0.6. The quality of the estimation was discussed through a practical identifiability study and quantified using an identifiability index. Tip radius and elasticity of the Berkovich indenter are two very relevant parameters to improve identifiability and correctly extract the plastic parameters of the behavior law. Scratch crack propagation resistance shows an evolution similar to those of Hp⁎/σY

Wear mechanisms of titanium and aluminium nitride coatings: A microtribological approach

International audienceMicrotribology experiments were carried out on a set of protective nanostructured Ti1-xAlxN (0 <= x <= 1) coatings, deposited by radio frequency magnetron reactive sputtering onto steel substrates and Si(100). Tests were carried out at room temperature using low applied loads and sliding velocities to prevent from surface oxidation. The surfaces were in contact against alumina to avoid the sticking of the counterpart, using a ball-on-disc reciprocating tribometer. Thus, these conditions allow the determination of the wear behaviour of the nitride layer itself. Film wear mechanisms were investigated from the evolutions of the friction coefficient and scanning electron microscopy observations. Moreover, two different models were used to characterise the coating according to x Al content: calculations of film fracture toughness K-IC from microindentation tests and crack propagation resistance CPRs from scratch experiments. By X-ray diffraction, growth directions of the crystallised domains of the nanostructured films are analysed. Combining the results obtained from the different mechanical tests, the film damages caused by friction stresses are presented as a function of composition and micro-and nanostructure of the films, which play a crucial role in the functionality of coatings. The amount of wear debris generated by friction is directly linked to the coating crack initiation resistance. The nature of wear debris, i.e. ductile or brittle, acting as a third body, has a major influence on the evolution of the thin film damag

Modélisation de l’indentation de revêtements multicouches Ti0,54Al0,46/Ti0,54Al0,46N

International audienceLe but de ce travail est de modéliser numériquement l’indentation de ces revêtements et de confronter les modèles aux résultats expérimentaux. La paramétrisation des modèles permettra de d’optimiser les conditions de dépôts à utiliser afin d’obtenir des revêtements performants

Investigation of Ti 0.54 Al 0.46 /Ti 0.54 Al 0.46 N multilayer films deposited by reactive gas pulsing process by nano-indentation and electron energy-loss spectroscopy

International audiencePhysical vapour deposition technology iswell suited to the deposition of advanced TiAlN-based coatings. Among these thin films, multilayer systems consisting of stacked layers of metallic Ti1−xAlx and nitride Ti1−xAlxNwith x around 0.5 are expected to have improvedmechanical properties with respect to single nitride layers of the same composition. A set of Ti0.54Al0.46/Ti0.54Al0.46N multilayer films with five different periods Λ (from 4 to 50 nm) were deposited using the reactive gas pulsing process (RGPP). This RGPP approach allows the deposition of TiAl-based alloy/nitride multilayer films by radio frequency reactive magnetron sputtering with a controlled pulsing flow rate of the nitrogen reactive gas. The coherent growth of the multilayer coatings, depending on the period, is checked by X-ray diffraction and the mechanical properties are determined by Berkovich nano-indentation and friction experiments. A model to describe the dependence of the indentation modulus M and the hardness HB on the penetration depth h, the period Λ, andthe film thickness ef is proposed. The indentationmodulus of themultilayer films (Mat h=0 and for ef ~ 1900 nm) is found to be in the range of 340 GPa bMb 525 GPa ≈M(Ti0.54Al0.46N). For a fixed penetration depth,Mfollows a Hall and Petch evolution as a function of the period (4 ≤ Λ ≤ 50nm). The Berkovich hardness, 25GPa b HB b 50 GPa, also presents the same kind of evolution, and for Λ b 16 nm (at h=0), HB N HB (Ti0.54Al0.46N)=33 GPa. Hence, a superlattice effect is clearly evidenced.Moreover,for the larger periods, the wear behaviour of these multilayered coatings seems to be dominated by the plastic deformation of the metallic layer. The multilayer coating of period Λ =10 nm, which exhibits a diffraction pattern typical of superlattices and favourable mechanical properties, is more precisely investigated. Transmission electron microscopy confirms the main growth of the film along the [111] direction, and the evolution of the bonding of nitrogen in the direction normal to the rough interfaces between Ti0.54Al0.46 and Ti0.54Al0.46N layers is specified by electron energy-loss near-edge spectroscopy. Nitride nano-grains are included in the metallic layer, which attests to the mixing of nitrogen into the layers. The structure of these nano-grains presents a progressive evolution into the layer and gradually acquires a TiN-like structure near the interface. For this Λ=10nm period, the indentation modulus and hardness for different penetration depths are weakly sensitive to the multilayer film thickness