Michel Ignat

Mechanical properties of W and W(C) thin films : Young's modulus, fracture toughness and adhesion

Abstract This contribution presents an investigation and the determination of the mechanical properties of tungsten (W) and tungsten–carbon (W(C)) single layer coatings deposited on steel substrates, in order to optimize a W/W(C) bilayer coating for mechanical applications. The coatings are first tested by three-point bending, to determine Young’s moduli. An acoustic emission detector, adapted to the bending device, allows the detection of the first stages of cracking in the coating and furthermore, by an analytical treatment, the determination of the fracture resistance. The adhesion of the coating on this substrate is analyzed from microtensile experiments, performed with a device adapted to a scanning electron microscope. A theoretical formalism is applied to analyze the stress redistribution in the cracked coatings. The elastic moduli of W and W(C) thin films are found to be very close to the bulk values of W. The determined fracture properties (toughness) ranged from 1 to 2.5 MPa m 1/2 and from 0.2 to 1 MPa m 1/2 for the W and the W(C) coatings, respectively. Their differences might be related to the film morphology. The adhesion of these coatings on stainless steel substrates appears good; experimentally no debonding was observed in either system.

Mechanical behaviour of hard PVD multilayered coatings

Abstract The aim of this work was to investigate the cracking behaviour and adhesion of tungsten–carbon-based multilayered coatings deposited on steel substrates by magnetron sputtering. Three-point bending experiments were performed on the coating-on-substrate systems until failure of the film. The systems were also strained uniaxially with a microtensile device adapted to a scanning electron microscope. The mechanical response is analysed from the evolution of the crack density in the coating and the fracture toughness. The results show that the rupture properties of the multilayered coatings are correlated to the film thickness and arrangement of the elementary layers. Scratch experiments on the systems revealed a strong adhesion of the multilayered coatings on steel substrates, and delamination at layer interfaces. Thus, graded coatings appear to be more attractive for mechanical applications.

Failure and adhesion characterization of tungsten-carbon single layers, multilayered and graded coatings

Abstract Hard multilayered and graded coatings based on a stacking arrangement of ductile layers and hard layers appear to be promising wear- or erosion-resistant coatings. To optimize the architecture of the protective coatings, the mechanical behaviour of the films has to be first investigated through basic mechanical tests before being subjected to specific tests. This paper reports a qualitative and comparative study of the adhesion and failure mechanisms of tungsten–carbon single layers, multilayered and graded coatings by means of scratch tests.

Adhesion behaviour of organically-modified silicate coatings on stainless steel

The mechanical properties of organically modified silicate coatings on stainless steel substrates were investigated, using nano-indentation and simultaneous in situ microtensile testing/optical microscopy. The load-displacement response and fracture behaviour is examined to ascertain the effects of different organic groups on the film properties and adhesion characteristics. The relationship between the morphology and mechanical properties of the films is discussed, and it is demonstrated that the mechanical response of the coatings is significantly influenced by the nature of the organic group attached to the ormocer precursor.

Cracking and Decohesion of Sol-Gel Hybrid Coatings on Metallic Substrates

Thin film coatings based on organically modified silanes were synthesized using sol-gel technology. Various mixtures of tetraethoxysilane and glycidoxypropyltrimethoxysilane precursors were used to produce sol-gel coatings on as-received and thermally oxidised copper, aluminium and titanium substrates. The mechanical properties and adhesion behaviour of the coatings were assessed using nano-indentation and microtensile testing, respectively. The relationship between the film structure and its mechanical response is examined. It is shown that the mechanical properties (hardness and Youngs modulus) of the coatings are influenced dramatically by the organic substituent and the presence of an oxide layer thermally grown on the substrate material prior to deposition plays an important role on the film/substrate adhesion behaviour.

The effects of particle pollution on the mechanical behaviour of multilayered systems

The mechanical stability of multilayered structures depends on the intrinsic properties of its layers, as on the external applied solicitations (thermal and/or mechanical). Moreover, the structural imperfections, as the ones generated during the deposition process of the layers, may also play an important role in the mechanical stability of the multilayers. For example, a continuous or a sequential laser irradiation on a multilayered mirror, can produce mechanical damage, which is nucleated at the imperfections of the multilayer. With the aim to study the mechanical damage associated to the imperfections of a multilayered structure, the response to an external stress on multilayered samples, with and without imperfections was analysed. The imperfections were obtained by the dispersion of fine diamond particles on polished substrates, before the deposition of the layers. The mechanically damage was progressively induced by pulling the samples in tension in a SEM. The critical parameters, characterising the behaviour and describing the damage are compared and discussed.

New approach for the critical size of nodular defects: the mechanical connection

The purpose of this paper is to determine the effect induced by the nodular defects in optical thin films under laser irradiation. A strong correlation between the size distribution of defects in optical thin films. The deposition parameters and the laser damage threshold is found. With this approach of the defects in thin films, it confirms that the size of a nodular defect is a critical parameter, while the density of the defects is not. The observations of damaged ares, performed by Scanning Electron MIcroscope (SEM), show that of the smallest ejected nodular defects associated with damaged zones, correspond to a critical mean size around 4 micrometers . A mechanical approach is also used to understand the role of the nodular defects under a mechanical solicitation. During the mechanical experiments it appears that depending on the nodular defects sizes, the initiation of cracking was more or less delayed. From the mechanical experiments, a critical size of about 4 micrometers is deduced. Besides, the analysis of these two different experiments points out that the laser damage induced by nodular defects is strongly related to mechanical fracture parameters.

Mechanical stability and adhesion of ceramic coatings deposited on steels

Abstract This paper presents the results of two sorts of deformation experiment performed on coating/substrate systems. The coating/substrate systems were constituted by coatings of titanium nitride and chromium carbide, deposited in both cases on steel substrates. The deformation experiments were cyclic bending tests on macroscopic samples with chromium carbide coatings, and straining experiments performed in a scanning electron microscope on samples with titanium nitride coatings. By the analysis of our experimental results we develop an attempt to correlate the mechanical stability of the systems with the interfacial adhesion, by taking into account the internal residual stresses as an adhesion parameter. For the samples with chromium carbide coatings, the evolution of internal stresses is detected from X-ray diffractometry and discussed in terms of the observed induced damaging mechanisms, in the cyclic tests. For the samples with titanium nitride coatings, we discussed the adhesion from the microstructural observations and from the critical parameters determined during the in-situ straining experiments.

Micro Tensile Tests on Aluminium Thin Films: Tensile Device and In Situ Observations

The results of micromechanical tensile experiments performed on thin aluminum samples are presented and discussed. The micro tensile test system and the design of the samples, based on finite element modeling (FEM), and their production by micromachining are briefly described. Some examples of the stress strain curves are presented. The Youngs modulus and critical parameters (flow and rupture stress and strains) are reported. The micro structural changes induced by the tensile experiment were observed during and after the testing by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).The results of micromechanical tensile experiments performed on thin aluminum samples are presented and discussed. The micro tensile test system and the design of the samples, based on finite element modeling (FEM), and their production by micromachining are briefly described. Some examples of the stress strain curves are presented. The Youngs modulus and critical parameters (flow and rupture stress and strains) are reported. The micro structural changes induced by the tensile experiment were observed during and after the testing by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Indentation and Fracture of Hybrid Sol-Gel Silica Films

Organic-inorganic hybrid thin films fabricated using sol-gel processing have many compelling properties that render them quite attractive for many applications, including optics, electronics, sensors and corrosion and scratch resistant films (Haas & Wolter, 1999; Sanchez et al., 2005). Organic-inorganic hybrid network materials have received much interest as transparent functional coatings on polymer substrates (Haas & Wolter, 1999; Haas et al., 1999a) and barrier coatings on metals (Metroke et al., 2001). Compared to glass, polymers such as polycarbonate (PC) and glycol bis(allyl carbonate) (CR-39) exhibit several advantageous physical and mechanical properties, such as high impact resistance and reduced weight, but also have the significant disadvantage of higher refractive index resulting in greater surface reflections as well as a much lower tolerance to abrasion. These drawbacks have limited their exploitation as a replacement to glass, especially for ophthalmic lenses where reflections and scratches on lenses can significantly obscure vision. The incorporation of a film or coating on glass or polymer can have immense benefits as is the case in the eyewear industry where several layers are deposited on polymer substrates to overcome substrate limitations (Samson, 1996; Schottner, 2001). By controlling the chemistry of the organic component incorporated in hybrid films, the physical and mechanical properties can be readily adjusted to realise specific attributes such as scratch resistance. Yet a vital reliability issue for film-on-substrate systems is the intrinsic mechanical properties of the film and adhesion to the substrate (Ignat et al., 1999).