Fracture study of organic–inorganic coatings using nanoindentation technique

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Abstract

The mechanical response of different coating-substrate systems are investigated using the nanoindentation technique. From the load–penetration depth curves, we determined the hardness Hc and the elastic modulus Ec of the coatings. Moreover, as the force increases, cracks, delamination and chipping can appear. These effects induce discontinuities on the indentation curves. Measuring crack lengths or calculating the dissipated energy during indentation allows the determination of residual stress in the coating and interface toughness. Two kinds of organic–inorganic coatings on different substrates (silicon and glass) are studied. The coatings were prepared by the sol–gel process and deposited using the spin–coating technique. The first coating is a mixture of methyltrimethoxysilane, colloidal silica and tetraethylorthosilicate and the second one is based on 3-(trimethoxysilyl)propyl-methacrylate. The first one reveals better general mechanical properties (lower residual stress, better adhesion, higher interfacial toughness) on silicon than on glass. For the second one, the elastic modulus and hardness are comparable with those of polymers. In contrast, coating toughness is lower.

Introduction

Hybrid organic–inorganic coatings find applications in different domains, particularly in optics with non-reflecting, anti-abrasion, or scratch resistant surfaces [1] and in optoelectronics with integrated optical circuits [2]. Such coatings are fabricated by the sol–gel process. Compared with others processing techniques, the great interest of sol–gel process is its relative tailoring simplicity. It is now well known that organic–inorganic hybrid precursors are very effective materials for such applications. However, for an industrial use, mechanical properties of the film and of the interface between film and substrates have to be known because they play a crucial role in coating efficiency and aging.

The nanoindentation technique is well known to permit the mechanical characterization of coating-substrate systems. The principle of the experiments is to indent the sample and to record the force as a function of the penetration depth. From the force–indentation depth curves, the hardness Hc and the elastic modulus Ec of the coatings are parameters classically obtained when the indentation depth is small compared to the coating thickness (about less than 10%). As the force increases, cracks, delamination and chipping can appear. These effects induce discontinuities in the indentation curves. More recently, Malzbender and de With [3], [4] showed that, by measuring cracks length or by calculating the dissipated energy during indentation, others mechanical parameters such as residual stress in the film, fracture toughness of the coating, fracture toughness of the interface between coating and substrate can be determined.

In this paper, two kinds of organic–inorganic coatings are studied. The first one is based on 3-(trimethoxysilyl)propyl-methacrylate and is used in integrated optical circuits fabrication. The second kind of coating is a mixture of methyltrimethoxysilane, colloidal silica and tetraethylorthosilicate and is used to make anti-abrasion films. The structure of these coatings are different: the first one is a copolymer with an inorganic part containing Si and Zr and an important organic network. The second one has a highly mineral structure with a network made only by siloxane bonds. Consequently, the mechanical behaviour of these systems is expected to be different. The aim of this work is to evidence such a difference by nanoindentation technique.

Section snippets

Experimental

Experiments are performed by using two kinds of hybrid organic-inorganic coatings. The first one (named A) contains 30% (weight) solids components and 70% solvents. The precursors are methyltrimethoxysilane (MTMOS, assay > 98%), colloidal silica and tetraethylorthosilicate (TEOS, assay > 99%). The weight amounts of MTMOS and colloidal silica are equal and the amount of TEOS is 2% (weight) of MTMOS quantity. The solvents are methanol (64%), diethylene glycol (34%), H2O (1%) and ethanol (1%). The

Results

Coatings thicknesses have been measured on cleaved sample using an optical microscope. The mean values are 5.1 ± 0.2 μm for A coating and 14.5 ± 0.2 μm for B coating.

Hardnesss and elastic modulus

As measurements have been performed at indentation depth lower than 10% of coating thickness, hardness and elastic modulus values do depend neither on substrate type nor on coating thickness. However, Hc and Ec values obtained for A1 system (silicon substrate) are larger than those of A2 system (glass substrate). Coatings have been heat treated at 250 °C and one possible explanation for lower mechanical properties is the diffusion of sodium ions from the soda lime glass substrate into the

Conclusion

Nanoindentation technique has been used to estimate mechanical properties of hybrid coatings on substrates. Hardness and elastic modulus have been determinate from indentation curves at small load values. At higher load values, coating toughness and residual stress as well as interface toughness were estimated from cracks, delamination and chipping occurring in the coating on the basis of geometrical and energetical analyses. The two kinds of coating which have been studied, have different

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