D. Chicot

Absolute hardness of films and coatings

Abstract Previously published hardness models are discussed and a new model is presented to obtain the absolute hardness of thin films or coatings. On the basis of the relative plastic deformation of the coating and the substrate this approach takes into account the role of the interface by the introduction of a function related to the substrate and to the film hardness coefficient. The model is verified for titanium nitride films on different substrate materials. For thick coatings (i.e.

Apparent interface toughness of substrate and coating couples from indentation tests

Determination of adhesive properties of coatings is one of the most important problems for the extension of the use of coated materials. Numerous studies are devoted to the research of new tests which have to face practical as well as scientific problems. In this work, we propose to evaluate adhesion of coatings by means of an apparent interfacial toughness. The interface indentation test provides a relation between the applied load (P) and the length of the crack (a) created at the interface between the coating and the substrate. Representing this relation in bilogarithmic coordinates shows that the relation is linear and may be expressed by the equation a = αPn where the exponent n is a function of the coating thickness. When the residual stresses due to solidification of the coating are removed by an appropriate annealing treatment, the straight lines corresponding to different thicknesses intersect at the same point. We have shown earlier that this point corresponds to the half diagonal of the indent and then to a limit (ac) for which no crack is formed by indentation. Therefore this point (critical point PC) may be considered as a criterion representative of the adhesive properties of the coating on its substrate. In order to give a usable value of these adhesive properties, we suggest to represent adhesion by an apparent interfacial toughness based on the critical load Pc. As the interfacial test is used to create and propagate a crack, some authors have proposed in a recent past to transpose the principle of indentation tests used to determine toughness of brittle materials. In this study, we expressed an apparent interfacial toughness in function of the critical point (Pc, ac), apparent elastic modulus E1 and apparent hardness H1 of the interface: Kca = 0.015Pcac3/2EH1 As the indentation is performed at the interface between the coating and the substrate, hardnesses and elastic properties of both the substrate and the coating are concerned by the indentation process. The (E/H)1 ratio had necessarily to be dependent on these properties. Considering the plastic deformations as well as the indentation diagonals into the substrate and into the coating, we may define the following relation: EH11/2 = EHS1/21 + HSHR1/2 + EHR1/21 + HRHS1/2 The critical load is representative of adhesive properties of the coating, then the apparent interfacial toughness should vary in the same way. We have verified this point on a chromium carbide coating thermally sprayed on various metallic substrates and on a stellite coating on an austenitic steel. The proposed model gives toughness values in good accordance with the critical indentation loads necessary to initiate a crack at the interface between the coating and its substrate. As a consequence, this apparent interfacial toughness may be considered as a criterion to represent adhesion of a coating on a substrate.

Residual stresses and adhesion of thermal spray coatings

Abstract Residual stresses generated in coatings during thermal spraying could have very different inside intensity and distribution, depending on the materials and processing conditions. As it is recognised that residual stresses play a major role in the adhesion of coatings, it is necessaryto evaluate precisely their influence. It is not possible to conduct these measurements directly, and no indication on how the stresses should be taken into account has been reported in the literature. Moreover, depending on the test used to evaluate adhesion, different volumes of the coatingcan participate in the delamination process. In order to take into account these observations, it is proposed to define two stress parameters related either to the stresses in the coating or to the stresses at the interfacial zone. In these conditions, it is possible to explain the variationin adhesion as a function of the coating thickness, i.e. to explain the maximum value obtained for the bonding strength deduced from tensile tests and the monotonic increase in adhesion toughness deduced from interfacial indentation tests.

Role of residual stresses on interface toughness of thermally sprayed coatings

In order to create and propagate a crack in the interface plane, Vickers indents were applied to polished cross-sections at the interface of Cr3C2/NiCr coatings obtained by the high velocity oxy-fuel spraying process. Using this procedure, it is possible to calculate an apparent interface toughness. For as-sprayed specimen, it was shown first that the interface toughness varies like a function of one over the square root of the coating thickness. The value extrapolated to an infinite thickness is assumed to represent the adhesive properties of the coating. Same tests were performed on annealed specimen for which it was expected a severe modification of the residual stresses state. In these conditions, an apparent interface toughness was found independent on the coating thickness. Moreover, this value is equal to the value extrapolated for the as-sprayed specimen. This very important and new result allows to modify the model for the interface toughness in order to take into account a stress parameter.

New developments for fracture toughness determination by Vickers indentation

Abstract Indentation is a traditional method used to determine the toughness of brittle materials. Different models are used for the calculation depending on the shape of the cracks that are initiated and developed as a result of the indentation. Recently it was observed that a transition between Palmqvist and median cracks is possible when increasing the indentation load. In the present study it is shown that this transition is not as sharp as is generally supposed, but is rather smooth. In these conditions standard calculation procedures cannot be applied. A new methodology is proposed here, which allows the calculation of a unique toughness value on the basis of the determination of the limits of the material cracking tendency.

Hardness measurements of Ti and TiC multilayers : a model

Abstract Microhardness measurements of thin films have to face the problem of the influence of the substrate on the measurement. Numerous models which take into account this behaviour are available in the literature. When the film is composed of several layers of different properties, it is more difficult to solve this problem because of the multiple influences of the different layers and the substrate. Little work on this subject has been already done; we propose here an extension to multilayers of two models: that of Jonsson and Hogmark, based on the relative deformed areas under the indent, and a model we have developed on the basis of plastically deformed volumes in the substrate and in the film. Starting from the hardness determination of Ti and TiC monolayer films, it is shown that it is possible to use either one or the other model to determine the composite superficial hardness of multilayer Ti–TiC films.

Indentation tests to determine the fracture toughness of nickel phosphorus coatings

In this paper, Vickers indentation was used to study the resistance to cracking of electroless nickel phosphorus coatings. For low indentation loads, cracking initiates only at the tips of the indent (primary cracking). It was shown that these cracks are of the Palmqvist type. For higher loads, new cracks, for which initiation sites are located on the edges of the indent, begin to form (secondary cracking). It was found that this change in the cracking mechanism occurs when the primary cracks reach the interface with the substrate. This result shows that the interface resistance to cracking is higher than the cohesion of the coating. In addition, it was possible to apply the same formulae whatever the cracking process, if the total number of cracks was divided by 4, like for the first cracking process. Values of fracture toughness Kc=1.5 MPa m1/2 for a 300 °C treatment and 2.1 MPa m1/2 for a 600 °C treatment were found, independent of the coating thickness.

Elastic properties determination from indentation tests

The knowledge of mechanical properties, and elastic modulus in particular, is necessary for a good understanding of the behaviour of materials subjected to static or dynamic loading. Accurate determination of elastic modulus is possible in the case of massive and homogeneous materials. For coatings the problem is more complicated but this determination is rendered possible using load and displacement sensing indentation tests or ultrasonic wave tests, for example. The purpose of this study is to determine elastic modulus of materials and specially that of coatings by analysing the morphology of indents obtained by the Vickers hardness test. During the withdrawal of the indenter, distortions of the faces of the indent are directly linked to the elastic properties of the material. Comparison between the true volume of the deformed indent and the ideal volume of the Vickers pyramid allows one, starting with Bulychevs relation, to achieve good estimations of the elastic modulus of massive materials as well as that of coatings. The model is first tested on massive material of known elastic properties (middle carbon steel, ferritic stainless steel and aluminium alloy), then it is applied to a chromium carbide coating obtained by hypersonic thermal spraying. Calculated elastic moduli are in accordance with literature data, except for the aluminium alloy, for which plastic flow around the indent is different from the other materials.

Influence of mechanical properties of tungsten carbide–cobalt thermal spray coatings on their solid particle erosion behaviour

Abstract The present investigation has been carried out in order to study the erosion wear behaviour of WC–Co base thermal spray coatings. WC–12Co and WC–10Co–4Cr coatings were deposited by means of high velocity oxygen fuel (HVOF) thermal spraying. The erosion tests were conducted at impact angles of 30 and 90° using SiC particles of ∼50 μm in diameter as erodent, at a velocity of 83·4 m s−1. It has been found that the erosion rate for both coated systems was higher when the test was carried out at an angle of 90°. The through-thickness residual stresses of the coatings, as well as the microstructural characterisation, allowed an explanation of the results and the erosion mechanisms in each case. It has been found that, under the experimental conditions carried out in the present study, the WC–10Co–4Cr coating exhibited a higher erosive wear resistance as compared to the WC–12Co coating.

Wear Behaviour of Silicon Carbide/Electroless Nickel Composite Coatings at High Temperature

Abstract The present paper reports the results obtained from sliding wear tests performed at both room temperature and high temperature (100 and 300°C) on silicon carbide/nickel composite coatings annealed at 400°C in argon. The coatings were deposited industrially on AISI 1020 steel discs by using a proprietary self catalysing chemical reduction process (Hardex). The tests were conducted without lubrication in argon, employing a ball on disc tribometer. A load of 5 N was used at a constant sliding speed of 0·1 ms-1. The mating pair was an alumina ball of 6 mm diameter. For the tests performed at 25°C, the effect of heat treatment contributed to an improvement in the coating wear resistance of nearly 135%, when compared to the as deposited conditions. It was determined that the set of the experimental conditions for the wear test performed at high temperature, i.e. the load used, the nature of the mating surface, and the low velocity of the test, added to a small coating thickness to particle size ratio, created very severe conditions, which caused coating removal. Wear rates nearly 23 and 11·6 times larger were obtained for the heat treated coatings tested at 100 and 300°C, respectively when compared with the wear resistance of the heat treated coatings tested at 25°C.